gtkconfig: set `accent-color` for GTK apps

Closed Fushan Wen requested to merge work/fuf/527accentcolor into Plasma/5.27

For GTK apps, expose the saved color as the one closest to the accents it officially supports.

This makes accent color consistent between Plasma and GNOME. That is to say, when a user logs out from Plasma and logs into GNOME, the color preference will be kept.

Depends on

Closes #5 (closed)

@lucabacci @tantalising @teams/vdg

Edited by Fushan Wen

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