Category popup fixes

Objectives:

* Make category popup show "Category" rather than missing message -
this is more similar to other popups like inspectors, which identify
what that thing is that you are working on
* Fix alignment of icon and label in category popup

Changes:

ve.ui.Widget.css
* Make popup title align properly with the remove button

ve.ui.MWCategoryPopupWidget.js
* Update message key

*.php
* Add new message

Change-Id: Ia8051125bbc9bde47ceb931e1ebf42b2955481ff
Merge "Category popup fixes"