LibGUI: Introduce widget content margins + improve splitters

A GUI::Widget can now set an optional content margin (4x0 by default.)
Pixels in the content margin will be ignored for hit testing purposes.

Use this to allow frame-like widgets (like GUI::Frame!) to ignore any
mouse events in the frame area, and instead let those go to parent.

This allows GUI::Splitter to react "sooner" to mouse events that were
previously swallowed by the child widgets instead of ending up in the
splitter. The net effect is that 2 more pixels on each side of a
splitter handle are now interactive and usable for splitting! :^)

Libraries/LibGUI/MultiView.cpp

@@ -39,6 +39,7 @@ namespace GUI {
 MultiView::MultiView()
 {
     set_active_widget(nullptr);
+    set_content_margins({ 2, 2, 2, 2 });
     m_item_view = add<ItemView>();
     m_table_view = add<TableView>();