Update VE core submodule to master (51a919d)

New changes:
618a773 Update OOjs UI to v0.1.0-pre (99ec31d9b9)
e651bd9 Update OOjs UI to v0.1.0-pre (0d358b167a)
0918a82 Use better unicorn image data
1eea699 Stage an empty comment node when inserting
e49a185 Fix height calculation in fragment inspector
26c8566 Stage a space when inserting a special character
480472c Localisation updates from https://translatewiki.net.
990e807 Update OOjs UI to v0.1.0-pre (db065e5a9f)
279031d Fix scroll jumping in Firefox when pasting at end of line
5519914 Handle paste events on the paste target

Change-Id: I760e0f31cbd20dc704a4131384d84d671520f339