mw.ViewPageTarget.init: Expose support.visualEditor

Refactor a few things to make it easier for scripts to see
not just whether mw.libs.ve is present but also whether it
allows the user to activate VisualEditor.

Change-Id: I50da8d9a260207d4ec1c43254dfe738f91386a9e
Merge "mw.ViewPageTarget.init: Expose support.visualEditor"