FlatControlLook: build fix

Change-Id: Iecc2e406dc5c97f9cd45c6ba1e4c279f63aa166c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7487
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Document new rgb_color apis, apply their usage.

Change-Id: I48f1bedd3abfb3b445c1c7090ce43504bcff4537
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7485
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Improve styles and visibility in dark mode

* Improve buttons & others
* Improve scrollbars visibility
* Improve darkmode

Change-Id: I8aae89cb659378def2e715938dd1f4e4f6455264
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7378
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

FlatControlLook: fix clipping

Remove ConstrainClippingRegion calls as they do not take into account
view transformations.
Clip drawing to the drawing rect, not the updated area.

Part of #12890

Change-Id: Icbc07f5431cf8ee826f28e6d5683135d1171d75a

FlatControlLook: Various clean-ups.

* Do not rely on symbols overriding other symbols, which is
not a safe assumption and not good practice either. Instead,
move all methods into FlatControlLook.

* Reorganize class structures somewhat.

* Remove unused variables.

control_look/decor: Add Flat style to haiku_extras package

* Works a bit better with dark themes
* Has some interesting alternative design elements
* MIT licensed

Change-Id: I6cfd1d4d05016fb014d515d94cff3b8c8d0298b2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4508
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>