Update VE core submodule to master (660ac11)

New changes:
af8f17d Add CollapseToFrom/To to Selection API
90d2085 Don't force focus after history action
56f2864 More flexible selection reporting in the debug bar
f01b696 Add content-heavy table to demo HTML
4b9857d Fix typo in getSelectionBoundingRect
781e24c Update OOjs UI to v0.1.0-pre (d74a46ca6a)
93b2dc5 Fix paste by never polling while pasting
b214e89 Fix theme on mobile-dist
aaccabe Replacing push.apply with newly created ve.batchPush in VE core
a00e846 Followup 2432cbf: add recursion guard to updateSlug

Change-Id: I75881c6d6de63589eba1be85e2ee0d0242cc4478