Embed window icon into executable using Qt resources

[wolfpackmars2]

This PR embeds the window icon directly into the executable using Qt resources, making the application more portable.

Changes:

• Added src/solvespace.qrc to embed the 48x48 icon PNG
• Updated CMakeLists.txt to compile Qt resources
• Modified guiqt.cpp to load icon from embedded resource instead of external file
• Works cross-platform (Windows, macOS, Linux)

Benefits:

• More self-contained and portable application
• No dependency on system-installed icon files
• Icon always available regardless of installation method

[wolfpackmars2]

This resolves #1681. The QT build will treat the window icon the same as the current GTK build, which embeds the pixmap data into the executable.

[phkahler]

@WickedSmoke Are you OK with this? He says it's a prerequisite to building an AppImage, but I'm guessing it may change your Linux packages for qt version (make them simpler?).

[WickedSmoke]

@phkahler That's fine, but it only sets the window manager icon (used in the task bar). Unfortunately packaging does not change since launch menus use the /usr/share/icons files.

I would put the .qrc file in the src/platform or res directory.

[phkahler]

I would put the .qrc file in the src/platform or res directory.

@wolfpackmars2 Looks like the 'res' folder is a good place for the .qrc Probably don't need to go deeper than that.

[wolfpackmars2]

I would put the .qrc file in the src/platform or res directory.

@wolfpackmars2 Looks like the 'res' folder is a good place for the .qrc Probably don't need to go deeper than that.

I made the change, and I agree.

I also renamed the file to 'qt-resources.qrc' to make it more obvious that this is a file that only affects the qt features. For devs who aren't familiar with QT.