Appearance: Make ColorPreview size font sensitive

AddGlue to separate color preview and color control instead
of growing the preview.

Change-Id: I6a21377cebc94a6f67c5528ba02d573f146a9b4c
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5500
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Drag and drop color from Appearance ColorPreview

You can drag a color square from the color preview and drop it
anywhere that accepts a color drop.

This allows you to use the color drop feature more readily
for example to change the desktop background color or change the
color of Deskcalc.

Also did some refactoring of ColorPreview class. Make it a BControl
which eliminates the enabled bool and invoker which both are handled
by the inherited BControl class.

Did some refactoring.
* Renamed a couple of class variables following convention
* Also renamed a couple of method parameters for the same reason

Don't call Draw() directly, this is frowned upon, instead use
Invalidate() so app server can draw at the appropriate time.

Rename ColorWell to ColorPreview in Appearance prefs.

No functional change intended.

I'd like to use the name "ColorWell" for a different type of
class eventually so I'm making room for it. ColorPreview is
arguably a better name for the class anyway.

Also did a style cleanup of the ColorWell => ColorPreview class at
the same time.

Rename ColorWell to ColorPreview in Appearance prefs.

No functional change intended.

I'd like to use the name "ColorWell" for a different type of
class eventually so I'm making room for it. ColorPreview is
arguably a better name for the class anyway.

Also did a style cleanup of the ColorWell => ColorPreview class at
the same time.