Terminal: graphical settings for configuring themes.

Implements #16835: Make colour schemes modular.
Implements #15936: Terminal settings should allow to drop a colour.

Change-Id: I1071c9285ba887e734af47b724f9de7f6c660581
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5950
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>