Update VE core submodule to master (31fe99c3f)

New changes:
31fe99c3f Localisation updates from https://translatewiki.net.

This restores translations for the 'visualeditor-pagemenu-tooltip'
message, which was moved to the submodule.

Change-Id: I9095d3d5450cd3517d70d590fde568e4f44672b8