[BREAKING CHANGE] Make ve.init.target an OO.ui.Element

Targets no longer take a $container argument, but generate their
own $element, which should be attached to the DOM by the caller.

Change-Id: Ie05cdd26ddebbe242b731bd277e5f613d08e0bee
Merge "[BREAKING CHANGE] Make ve.init.target an OO.ui.Element"