Split ve.ui.Context into DesktopContext and MobileContext

DesktopContext handles more advanced positioning of the context menu and
inspectors on desktop. MobileContext is more basic and can be used on
platforms where context menus are not present (mobile/tablets).

Change-Id: Id3f5f3f39978836f084f7c7e15505e4b6aeb9b44
Merge "Split ve.ui.Context into DesktopContext and MobileContext"