Update VE core submodule to master (c8201dd)

New changes:
ff237d4 Fix z-indexes in core
e88d43e Localisation updates from https://translatewiki.net.
cf61803 Consistently use ve.ui.WindowManager everywhere
f9dfdb8 Update OOjs UI to v0.1.0-pre (23565e7519)
f79f7e3 Update OOjs UI to v0.1.0-pre (8f8896196f)
c8201dd Update OOjs UI to v0.1.0-pre (9ed4cf2557)

Local changes for the breaking change to OptionWidget and sub-classes.

Change-Id: Ife6abd312d4dc97be67cb84eea4cb9c6a0a31b1d