LibGUI: Move table view headers into their own widget

This patch introduces the HeaderView class, which is a widget that
implements the column headers of TableView and TreeView.

This greatly simplifies event management in the view implementations
and also makes it much easier to eventually implement row headers.

Libraries/LibGUI/Forward.h

@@ -43,6 +43,7 @@ class DropEvent;
 class FileSystemModel;
 class Frame;
 class GroupBox;
+class HeaderView;
 class HorizontalBoxLayout;
 class HorizontalSlider;
 class Icon;