Big change deriving BLayout from BLayoutItem, and allowing viewless BLayouts.

a few highlights:
* BLayout now derives publicly from BLayoutItem
* Added BAbstractLayout class, which our layouts now derive from
* updated layout builders to avoid creating views when they don't need to
* updated layout classes
* updated AboutSystem to fix a little regression
* more details on #6407
* please tell me about any regressions, I've tried to find them all, but some
  may have slipped by.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@38207 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/apps/aboutsystem/AboutSystem.cpp

@@ -579,6 +579,7 @@ AboutView::AboutView()
 	const float offset = 5;
 
 	SetLayout(new BGroupLayout(B_HORIZONTAL));
+	SetViewColor(ui_color(B_PANEL_BACKGROUND_COLOR));
 
 	BLayoutBuilder::Group<>((BGroupLayout*)GetLayout())
 		.AddGroup(B_VERTICAL)