Update VE core submodule to master (15cce1d)

New changes:
235faf9 Try a bit harder not to create empty text nodes
72f9aff Followup 355be3ed: fix getOffset() behavior inside block aliens
c81dd57 Trigger handleDelete inside onCut
edb4c60 Only block language change if dir hasn't changed
15cce1d Localisation updates from https://translatewiki.net.

Change-Id: Ic205fd2ae9ddc3f870b74a49bfc02fea9a14546a