Modifications:
VisualEditor.php
* Added links to new widgets
VisualEditor.i18n.php
* Added placeholder text for category input
ve.ui.Widget.css
* Added styles for new widgets
New:
ve.ui.MWMetaDialog.js
* Create category widget with categories from dm.
* Listen to metaList for insert and remove events
** insert / remove bound methods to be improved upon additional meta groups
* Add listeners to mwCategoryWidget for new categories and updates
ve.ui.MWCategoryWidget.js
* Top-level category editing widget
ve.ui.MWCategoryItemWidget.js
* Items within a category widget
ve.ui.MWCategoryInputWidget.js
* Input for new categories, handles menu and API requests
ve.ui.MWCategoryPopupWidget.js
* Mini-inspector for a category item
Bug: 39597
Change-Id: I5eafaa484a1924a566d3a1ee1d869293089d0ecf
