Move welcome screen bg image to gui/ui/images so that it gets packaged

Set stylesheet in code with full path to image

Author: Dimitar Tasev

images/welcome_screen_background.png renamed to mantidimaging/gui/ui/images/welcome_screen_background.png

@@ -6,20 +6,13 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>600</width>
-    <height>400</height>
+    <width>510</width>
+    <height>340</height>
    </rect>
   </property>
   <property name="windowTitle">
    <string>About Mantid Imaging</string>
   </property>
-  <property name="styleSheet">
-   <string notr="true">#WelcomeScreenDialog {
-border-image:url(images/welcome_screen_background.png);
-min-width:30em; min-height:20em;
-max-width:30em; max-height:20em;
-}</string>
-  </property>
   <layout class="QVBoxLayout" name="verticalLayout">
    <item>
     <widget class="QLabel" name="version_label">

mantidimaging/gui/ui/welcome_screen_dialog.ui

@@ -1,6 +1,8 @@
 # Copyright (C) 2020 ISIS Rutherford Appleton Laboratory UKRI
 # SPDX - License - Identifier: GPL-3.0-or-later
 
+from mantidimaging.core.utility import finder
+import os
 from PyQt5.QtWidgets import QLabel
 
 from mantidimaging.gui.mvp_base import BaseDialogView
@@ -9,6 +11,12 @@
 class WelcomeScreenView(BaseDialogView):
     def __init__(self, parent, presenter):
         super(WelcomeScreenView, self).__init__(parent, "gui/ui/welcome_screen_dialog.ui")
+        base_path = os.path.join(finder.get_external_location(__file__), finder.ROOT_PACKAGE)
+
+        bg_image = os.path.join(base_path, "gui/ui/images/welcome_screen_background.png")
+        self.setStyleSheet("#WelcomeScreenDialog {"
+                           f"border-image:url({bg_image});"
+                           "min-width:30em; min-height:20em;max-width:30em; max-height:20em;}")
         self.presenter = presenter
 
         self.issue_box.setVisible(False)