gdk/wayland: Support new `high-contrast` key
We now have a boolean setting that determines whether the high-contrast
theme should be used. Support it by automatically setting the existing
gtk-theme-name
and gtk-icon-theme-name
properties when enabled.
With that, it is no longer necessary to change the regular theme settings for high-contrast, so toggling between high-contrast and a non-default theme finally works reliably.
Same as !4271 (merged), but for GTK3.