Update VE core submodule to master (1835848)

New changes:
6b5a276 ve.ui.FormatTool: Update heading size CSS for new OOUI version
2dd5e9c Fix getAnnotationsFromRange when starting with an element
d7db9af Localisation updates from https://translatewiki.net.
3aa1c88 Update OOjs UI to v0.1.0-pre (44db8292bf)

Change-Id: Id5a465af1f14f9e0371d7672d04a7430bcea98d6