wayland: add wayland-csd-adwaita-notitlebar feature flag

[timon-schelling]

sctk-adwaita has a new hide_titlebar feature in 0.11. This pr adds a feature flag to enable that for wayland csd.

Usecase:

• Allows using frame provided by sctk-adwaita with a custom or no title bar.
• We would like to use something like this for the upcoming graphite desktop app on wayland (see image).

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

[kchibisov]

Can it be additive over notitle? I'd assume that notitle is the one with bar, but no text, so this one should depend on notitle.

[timon-schelling]

Didn't test that but should work. What do you think about the name? Something like frame-only would also work.

This could also be implemented as a option on the Wayland Windows Attributes (because it is not just a mapping to sctk-adwaita feature flag like notitle) would you prefer that?

[kchibisov]

Don't have a strong preference, whatever makes it clear for downstream from your standpoint. I just want it be additive logically on top of existing one.

[timon-schelling]

I just want it be additive logically on top of existing one.

Done. And moved the hide_titlebar call to the function that creates the sctk FrameConfig, will work after changing decoration flag.