Update VE core submodule to master (db78bdd)

New changes:
69ecc8e Remove MW-specific mobile hack from core
e556440 doc: Use wildcards for Desktop/Mobile …Context and …Surface for simplicity of extension

Local change:
Bring in MobileContext hack to filter out all tools except links and citations from core where
it didn't belong.

Change-Id: Ica837abd45f0ff02b49a44da617bc1fd8e9872d4