Update CEF Version to 138.0.21

[wavecommander]

I did not test this on Windows.
Windows may be broken before and/or after this change for all I know

This PR updates the CEF version used to the latest: 138.0.7204.101.
There was only a small change to the function signature of CefClient::OnBeforePopup that prevented compilation with the newer version and needed to be updated in the WebviewHandler subclass.

[wavecommander]

@1056824847

flutter 3.32 version runs abnormally, flutter 3.29 runs normally

How so? What results did you find?
I did make this PR with version 3.32 installed.

[wavecommander]

I reverted adding the unnecessary dependency on GTKMM and updated the target_modifier/tools/* scripts to check for files like main.cc in both linux/ and linux/runner/.

The GitHub Action should now complete successfully:
https://github.com/wavecommander/webview_cef/actions/runs/16503468721/job/46667989963

[Copilot]

Pull Request Overview

This PR updates the CEF (Chromium Embedded Framework) version from 130.1.2 to 138.0.21 to use the latest available build. The update required a breaking change fix to the OnBeforePopup function signature which now includes an additional popup_id parameter.

• Updates CEF binary URLs and versions across Linux and Windows platforms
• Fixes compatibility with new CEF API by adding popup_id parameter to OnBeforePopup method
• Improves shell script robustness with better file path handling

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
third/download.cmake	Updates CEF binary URLs and version strings to 138.0.21 for Linux ARM64/x64 and Windows x64
linux/target_modifier/tools/*.sh	Enhances file existence checking with fallback to parent directory
linux/target_modifier/modify_target.sh	Updates file paths to include /runner/ subdirectory
linux/CMakeLists.txt	Switches from PkgConfig::GTK to direct GTK library linking
lib/src/webview_manager.dart	Adds null safety check for injectUserScripts parameter
common/webview_handler.h	Adds popup_id parameter to OnBeforePopup method declaration
common/webview_handler.cc	Adds popup_id parameter to OnBeforePopup method implementation