Begin writing out settings to GTK4

GTK4 seems to be coming out later this year (Plasma 5.21 should release after GTK4 releases), and there doesn't seem to be any major changes coming up in regards to how configuration will be handled.

As such, this patch begins writing out to GTK4 configuration files.

Code has been refactored to reduce duplication of similar code between GTK3 and GTK4, and elements shared between both have dropped the GTK3 prefix.