ve.ui.LookupWidget

Refactoring of externally sourced suggestions for text inputs.

*.php
* Added links to new file

ve.ui.InputLabelWidget.js
* Changed to focus input element, not wrapper div

ve.ui.InputWidget.js
* Fixed incorrect documentation

ve.ui.LookupInputWidget.js
* New mixing that abstracts placing a menu of options below a text input
  and filling it with data from an external source

ve.ui.MenuWidget.js
* Fixed to get reference to input element, no wrapper div

ve.ui.MWLinkTargetInputWidget.js
* Moved pending and lookup functionality to mixing
* Implemented menu population using only matching pages, rather than a
  combination of that and page existence checks (fewer API calls)

ve.ui.TextInputMenuWidget.js
* Added configurable container to render underneath, rather than assuming
  this.input.$
* Added auto-position-on-window-resize functionality
* Fixed frame position correction to ensure that it only is used when the
  overlay is in a different frame from the container to position
  underneath

ve.ui.TextInputWidget.js
* Added placeholder text feature

Change-Id: If5ed1b64fd15982807691ce8bb0362970633108a
Implement ve.dm.Surface#truncateUndoStack

ve.dm.Surface.js
* Allow manual truncation of the undo stack to prevent redoing something
  that's been undone

Change-Id: I17534d6724fc2b325152cb2f665c6816f44232c1
Remove singular adding for ve.ui.GroupElement

By making the API for adding/removing/clearing always plural, it greatly
simplifies the interfaces and reduces function call overhead in most
situations.

Change-Id: Ia8f23a373a01a8f6d5081587a591563e4f25ea42
Category editing

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
Merge changes I5eafaa48,Ia8f23a37,I17534d67,If5ed1b64

* changes:
  Category editing
  Remove singular adding for ve.ui.GroupElement
  Implement ve.dm.Surface#truncateUndoStack
  ve.ui.LookupWidget