Support title background color on wayland

[tonystr]

Description

Hello & thank you for maintaining this wonderful repository,

sctk-adwaita supports setting custom colors for the titlebar with ColorTheme (source). So it should be possible to support theming the titlebar with a new method like WindowExtWayland::set_title_background_color() similar to the existing WindowExtWindows::set_title_background_color()?

This would be very useful for applications like Neovide, which supports titlebar theming for windows and MacOS, but not wayland.

If this isn't currently possible to implement, what are the obstacles hindering it?

Relevant platforms

Wayland