Qt5 source AppImages

[JulianGro]

This PR enables using Qt5 source packages for building AppImages.

Known issues:

• Fcitx support isn't shipped in those Qt5 source AppImages, since the Qt Fcitx plugin comes form Fcitx and not Qt itself. (I wouldn't be against making a release without Fcitx support and just putting this into the changelog as “known issue” and telling people to skip this version if they want to use Fcitx.)
• Packaging currently fails without manual intervention, since patched linuxdeploy-plugin-qt is required. Don't fail on non-existent plugin directory. linuxdeploy/linuxdeploy-plugin-qt#212
• Builds might fail due to Conan Center's binary cache for glib not being compatible with Ubuntu 22.04 or newer (so basically every Linux system). This can be worked around by passing --build="glib/*" to ignore the binary cache.

Fixes: #2078

The relevant recipe change is here: https://github.com/overte-org/overte-conan-recipes/pull/50/changes

If anyone wants to try the AppImage before the CI pipeline gets fixed: https://data.moto9000.moe/temp/Overte-Dev-2026-02-26-0-x86_64.AppImage

[zedwick]

I've tested the linked AppImage and can confirm it fixes #2078 for me

[JulianGro]

There are multiple issues we would need to work around to get this to build on the CI pipeline. Before I open some huge can of worms, I took out the commit that enables use of this package on the CI pipeline. It can be manually selected using -o &:qt_source=source.

I can just manually create AppImages again, if we want to make a release with it.