Draft: extras: build: add flatpak build support

This adds support for Flatpak building. With mostly using our contribs on top of KDE runtime.

Run extras/packages/flatpak/build.sh to build the Flatpak package.

This is a draft. Although it is almost complete, there are some issues to be fixed. Probably something to do with QT (This QT is provided by KDE runtime by the way.)

What works:

• The first time running wizard/dialog.
• Running videos with VLC, for example: flatpak run org.flatpak.Vlc filename.mp4 runs the video.

What does not work:

• The main interface does not load. A transparent unusable window is launched only.

Related to: #28356