Split ve.ui.Surface into DesktopSurface and MobileSurface

This will allow us to set a custom Context (and probably other things
too) for desktop/mobile.

Remove Suface.$localOverlayMenus which is only used in ve.ui.LinkInspector
and attach link inspector directly to the context instead. This will
prevent MWLinkTargetInputWidget's lookup menu from scrolling together
with the surface on mobile in further patches.

Change-Id: Iee68ea9534531a21fb9aeae08642856ad4a511bc