Several performance improvements.

Added events: Node::onIdChange, UIWidget::onClassChange, UIWidget::onTagChange.
UIItemContainer is now a widget (required for CSS).
Fixes in UIListBox and UITable.
CSS::StyleSheetStyle is now stored as a shared pointer.
Fixes in UICheckBox, UIRadioButton, UISlider, UIScrollBar, UITab, UIWinMenu, and more.
Better debug tooltips (now shows padding and margin).
Fix in UISceneNode::loadLayoutNodes, ensures that the current SceneNode is the one that it's loading.

bin/assets/ui/breeze.css

@@ -287,8 +287,6 @@ ScrollBar {
 ScrollBar::hback {
 	min-height: 4dp;
 	border-radius: 3dp;
-	margin-left: 4dp;
-	margin-right: 4dp;
 	background-color: transparent;
 }
 
@@ -296,8 +294,6 @@ ScrollBar::vback {
 	min-width: 4dp;
 	border-radius: 3dp;
 	background-color: transparent;
-	margin-top: 4dp;
-	margin-bottom: 4dp;
 }
 
 ScrollBar::vslider {