Outline of labels/strings drawn to desktop

* Rather than duplicating the decision taking logic involving wheter or not to draw
the outline or glow in every replicant, update be_control_look to make it more
generic.
* The Monitoring of the background preferences is now only done in Tracker (where it
was already being done).
* Add a BControlLook::B_IGNORE_OUTLINE flag to avoid this new behaviour.
* Remove that said logic from ActivityMonitor and use be_control_look.
* Use the ignore flag in DeskCalc to avoid the outline in its case.

Should fix #7716, #7291.

src/apps/deskcalc/CalcView.cpp

@@ -416,6 +416,7 @@ CalcView::Draw(BRect updateRect)
 					flags |= BControlLook::B_BLEND_FRAME;
 				if (key->flags != 0)
 					flags |= BControlLook::B_ACTIVATED;
+				flags |= BControlLook::B_IGNORE_OUTLINE;
 
 				be_control_look->DrawButtonFrame(this, frame, updateRect,
 					fBaseColor, fBaseColor, flags);