Update VE core submodule to master (0709218)

New changes:
20c220a Remove unused CSS classes
9bdcc65 Comment icon positioning
e50e790 [BREAKING CHANGE] Fix ve.ui.Overlay classes
1ac4927 ve.ce.TextNode: Use getChar() helper
29fd0f8 ve.ce.TextNode: Simplify #getAnnotatedHtml logic
18336eb ve.ce.ResizableNode: Run find() once instead of 4x in showHandles()
febf125 Use getDescription for title attribute
ab3cb6c Localisation updates from https://translatewiki.net.

Local changes:
* Rename overlay class rules

Change-Id: Ia90b6afbd4abdf3267f4ccaac0b2a7d4101ccc7b