[RelEng] Add release preparation script to update the Splash-screen

The script will be called by the pipeline to prepare a new development
cycle and automates the update of the splash-screen for the new cycle.
It requires that all updated images are available in advance of the
preparation.

Author: HannesWell

platform/org.eclipse.platform/futureSplashScreens/ReadMe.md

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+## Description
+
+This directory contains the Splash-screen and About-dialog images of **future** versions of Eclipse.
+It serves as a kind of staging area and is not included in the built bundle.
+These images will be moved to their final location by the `prepareNextDevCycle.sh` script, which is called during the preparation of the development of the corresponding release.
+Therefore the image(s) for a corresponding release must be available in this directory **BEFORE the DEVELOPMENT of that release is PREPARED** and the following requirements apply:
+
+- The Splash-screen images **must** be named according to the following schema:
+`splash_${NEXT_RELEASE_YEAR}.png`
+for example `splash_2025-12.png`.
+
+- The About-dialog image (and it's high-DPI version) of a future version must be added to this directory, if changed, and be named
+`eclipse_lg.png` respectively `[email protected]`
+
+While there is a dedicated Splash-screen image for each release (containing the version of the release),
+the About-dialog image is the same for multiple releases as long as they share the same Splash-screen base style/template.
+
+The Splash-screen and About-dialog images of the currently developed and past Eclipse versions don't need to be included in this directory.
+
+## Providing new Splash-screen images for a future release
+
+Due to the requirements described above, the images for a new Splash-screen base style/template must be added to this folder
+before the development of the first release that is intended to use them is prepared.
+For example if a Splash-screen style/template is used for 2026-06 ff., the new images should be added at latest near the end of the 2026-03 development.
+When a new Splash-screen style/template is introduced, typically the Splash-screen images for four consecutive releases are added to this directory,
+following the naming requirements described above.
+The About-dialog image(s) is typically the same for these four consecutive releases, i.e. as long as the Splash-screen style/template is unchanged.
+
+The Splash-screen and About-dialog images of the currently developed and past Eclipse versions in this folder are irrelevant and should be removed.
+In fact the mentioned `prepareNextDevCycle.sh` script will _move_ the images to their production location when they become effective, i.e. when a new release is prepared.
+This behavior clears this directory automatically over time.
+
+To test the result of a future release preparation, you can run the following bash snippet (adjust the numbers accordingly):
+````
+export NEXT_RELEASE_VERSION=4.39
+export NEXT_RELEASE_NAME=2026-03
+./prepareNextDevCycle.sh
+```
+**Please make sure to not submit the resulting commit!**

platform/org.eclipse.platform/splashscreens2025/Splash Screen_2026-03.png renamed to platform/org.eclipse.platform/futureSplashScreens/splash_2026-03.png

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+#!/bin/bash -xe
+
+#*******************************************************************************
+# Copyright (c) 2025, 2025 Hannes Wellmann and others.
+#
+# This program and the accompanying materials
+# are made available under the terms of the Eclipse Public License 2.0
+# which accompanies this distribution, and is available at
+# https://www.eclipse.org/legal/epl-2.0/
+#
+# SPDX-License-Identifier: EPL-2.0
+#
+# Contributors:
+#     Hannes Wellmann - initial API and implementation
+#*******************************************************************************
+
+# This script is called by the pipeline for preparing the next development cycle (this file's name is crucial!)
+# and applies the changes required individually for Eclipse-Platform.
+# The calling pipeline also defines environment variables usable in this script.
+
+pushd 'platform/org.eclipse.platform'
+
+if [ ! -f splash.png ] || [ ! -f eclipse_lg.png ] || [ ! -f [email protected] ]; then
+	echo 'Expected target Splash-screen or branding images are missing'
+	# Probably the naming or file format of any of these files has changed and this script wasn't adapted.
+	exit 1
+fi
+
+mv -f futureSplashScreens/splash_${NEXT_RELEASE_NAME}.png splash.png
+if [ -f 'futureSplashScreens/eclipse_lg.png' ]; then
+	# About-dialog image is usally the same for multiple releases
+	mv -f futureSplashScreens/eclipse_lg.png eclipse_lg.png
+	mv -f futureSplashScreens/[email protected] [email protected]
+fi
+popd
+
+git commit --all --message "Splash Screen for ${NEXT_RELEASE_VERSION} (${NEXT_RELEASE_NAME})"