[Linux] Implement native color picker.

[bruvzg]

Implements native color picker on Linux.

• Fixes issues with color picker on XWayland (see Fix legacy color picker crash. #101513).
• Allows picking color outside window on Wayland.
• Show also look a bit better on native X11.

Screencast.From.2025-01-14.18-23-18.mp4

[YeldhamDev]

Tested on KDE Plasma, works as expected.

[Calinou]

Tested locally on KDE X11 6.2.5 (with and without single-window mode), it doesn't seem to do anything when clicking the pick color button:

color_picker_not_working.mp4

In comparison, it worked on 4.4.dev 24d7451.

[syntaxerror247]

I'm not sure about adding a new method in DisplayServer. Can this be implemented for other OSes too?
Android doesn't have a native widget for it.

[bruvzg]

Added availability checks for org.freedesktop.portal.Screenshot (same as #101812).

[bruvzg]

I'm not sure about adding a new method in DisplayServer.

D-Bus/FreeDesktop portal is already initialized and polled as part of DisplayServer, so it's a logical place to add it to reuse a lot of existing code.

Can this be implemented for other OSes too?

A far as I can tell, no. None of the other OSes have anything like this (macOS and Windows have only full color picker dialogs), but this is likely the only way to pick from a screen on Wayland.

[Riteo]

I tested this PR on sway, and it worked fine even on my clunky KISS Linux setup! I'll do a final approval once #101812 is merged, as this PR includes that.

[Riteo]

Tested once again, with successful results, on my funky KISS Linux laptop (sway using xdg-desktop-portal-wlr backend). Code checks out.

LGTM!

[Repiteo]

Thanks!