feat: Install adw-gtk3 on first run

[daudix]

Feature description

Currently, Gradience makes user to manually install Flatpak and system adw-gtk3 theme, it would be much simpler for user if Gradience could do this by itself.

Since we now have flatpak-spawn permission, we can run flatpak install on host to install adw-gtk3 Flatpak, and fetch the latest adw-gtk3 release to .local/share/themes with symlink to .themes for compatibility with Fedora Flatpaks.

If we want to go even fancier, preferences could have adw-gtk3 updater that updates system theme, with auto-check for new release and sending notification to user with suggestion to update (maybe even auto-run could be implemented with new background portal, but notification when Gradience is open is more than enough)

Additional Context

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[lavafroth]

We might add something like "base":"org.gtk.Gtk3theme.adw-gtk3" to the flatpak manifest. Refer to example usage of a base app.