ve.ui.PopupWidget visibility fixes

Objective:

* Context popup would stop opening sometimes "mysteriously" which ended
  up having to do with the automatic closing on blur functionality
  added to popups for use in the category popup widget
* Mousedown event canceling was being applied a little too widely, and
  was causing popup widgets to not allow child elements to be focused
  (unless they contained an iframe, like an inspector)

Changes:

ve.ui.Context.js
* Make use of the popup's show and hide methods within the inspector

ve.ui.MWCategoryPopupWidget.js
* Override autoClose option for category popup widgets

ve.ui.PopupWidget.js
* Add autoClose option to popup widgets
* Move event handler to the top of the methods (convention)
* Only bind blur event if autoClose is enabled
* Inline the getFocusedChild method

Change-Id: I22aedb5fbd51b327ea7ce2ecdd6123e79cbebb9c
Merge "ve.ui.PopupWidget visibility fixes"