Category moving improvements

Changes:

ve.dm.MetaList.js
* Auto-initialize index to the end of the list (like the docs claim) even when not providing an offset

ve.ui.MWCategoryInputWidget.js
* Cleanup fix for not offering to move the last category to the end

ve.ui.MWCategoryWidget.js
* Allow pressing cancel on the meta dialog after moving a category to the end without crashing

Change-Id: I45059bda022639f7942f1799a42ecf8a4d962df0
Merge "Category moving improvements"