Update VE core submodule to master (520299e)

New changes:
dd15f23 Split ve.ui.Surface into DesktopSurface and MobileSurface
16283f4 Add OOjs UI's sco.json i18n file
ef94038 Split ve.ui.Context into DesktopContext and MobileContext

Minor adjustments to point to desktop and mobile Surface or Context.

Change-Id: I7cf6f99a5a1216a28a7146afcd4deb68c7eac38e