Enable XWayland only for skinned UI (part 1)

[jlindgren90]

The newer (GTK and Qt) UIs are mostly usable with native Wayland. Undocked plugins are still problematic, since positions are not saved/restored, but (1) keeping them docked is a workaround and (2) maybe it's time to let the problem be more visible so it gets solved?

Needs accompanying change on plugins side.

[radioactiveman]

As suggested in https://github.com/orgs/audacious-media-player/discussions/1664 it would be nice to have a configuration option for Xwayland.

My idea for this would be:

• Add a libaudcore bool setting (I see no need to have separate ones for GTK and Qt).
• Add a corresponding checkbox in Audacious Settings -> Advanced -> Compatibility,
  allowing users to opt-in for Xwayland support. And not just for the Winamp interface but also gtkui/qtui.
• When selecting or starting the Winamp interface on Wayland show an error dialog (similar as in your PR #194) and ask the user to enable the Xwayland checkbox and restart Audacious. Or enable the checkbox automatically when pressing the Restart button.

What do you think?

[jlindgren90]

I'm fine with adding configuration option but I think we still need to force XWayland for the skinned UI, it's basically unusable otherwise.