Update VE core submodule to master (a2ac637)

New changes:
2074a5d Fix application of slug focus classes
9f4ab6a Have Protected/ResizableNode respond to rerender event
9d7a290 Rerender focusedNode after slug collapse
de7d9ab Use en-us spelling 'focused'
7fae4c2 Add missing returns documentation to getSurfaceFragment
5177898 Remove unneeded assignment
a1c12c2 Localisation updates from https://translatewiki.net.
5648139 Localisation updates from https://translatewiki.net.
548728a Restore error box to VE demo page
684e55d Localisation updates from https://translatewiki.net.
3c4a081 ve.ce.Node: Change slug color from darker grey to lighter blue/grey

Change-Id: Icb20d78bb1fe65dd9107e43317148d4990bcfc6b