Allow better toolbar customisation for VisualEditor

The toolbar in very desktop vector skin specific
In mobile we want to have more control over the toolbar,
and its placement.
* Thus make setUpToolbar abstract
And move the function to ViewPageTarget

* Introduce ve.init.mw.Target.static.iconModuleStyles to
allow the use of different icons
* Update the mobile toolbar to only have B and I tools

Change-Id: I4c72b4b9128b3a74de8b8b5bce7664fbb315216b