Major UI refactoring and improvements

Objective:

Refactor UI widgets, improve usability and accessibility of menus, general cleanup and style improvements.

Extras:
Fixed documentation in a few other files to make descriptions of jQuery event arguments more consistent, classes inherit correctly, and made use of the @cfg functionality in jsduck.

Changes:

.docs/config.json
* Added window, HTMLDocument, HTMLElement, DocumentFragment and XMLHttpRequest to externals, so jsduck doesn't throw warnings when they are used

demos/ve/index.php, modules/ve/test/index.php, VisualEditor.php
* Moved widgets above tools (since tools use widgets)

demos/ve/index.php
* Refactored widget initialization to use options
* Renamed variables to match widget names

ve.init.mw.ViewPageTarget.css
* Adjusted text sizes to make widgets work normally
* Added margins for buttons in toolbar (since button widgets
don't have any)
* Removed styles for init buttons (button widgets now)

ve.init.mw.ViewPageTarget.js
* Switched to using button widgets (involved moving things around
a bit)

ve.ui.LinkInspector.js, ve.ui.MWLinkInspector.js
* Renamed static property "inputWidget" to
"linkTargetInputWidget" to better reflect the required base class
for the properties value

icons.ai, check.png, check.svg
* Added "check" icon, used in menu right now to show which item
is selected

ve.ui.Icons-raster.css, ve.ui.Icons-vector.css
* Added check icon
* Removed :before pseudo selectors from most of the icon classes (not need by button tool anymore, makes them more reusable now)

ve.ui.Tool.css
* Adjusted drop down tool styles so menu appears below, instead
of on top, of the label
* Adjusted paragraph font size to better match actual content
* Updated class names to still work with menu widget changes
(items are their own widgets now)
* Updated selectors as per changes in the structure of button tools

ve.ui.Widget.css
* Added styles for buttons and menu items
* Adjusted menu styles

ve.ui.*ButtonTool.js
* Added config options argument passthrough

ve.ui.ButtonTool.js
* Moved var statement to the top inside constructor
* Switched to using "a" tag to get cross-browser :active support
* Added icon to inside of button to make icon styles more reusable
* Removed disabled support (now provided by widget parent class)

ve.ui.FormatDropDownTool.js
* Updated options initialization to construct menu item objects
* Modified handling of items to account for changes in menu and
item classes
* Optimized onUpdateState method a bit, adding early exit to
inner loop

ve.ui.ButtonTool.js, ve.ui.DropdownTool.js, ve.ui.Context.js,
ve.ui.Frame, ve.ui.Tool.js, ve.ui.Widget.js
* Added chain ability to non-getter methods

ve.ui.DropdownTool.js
* Removed items argument to constructor
* Updated code as per changes in menu class
* Fixed inconsistent naming of event handler methods
* Removed item event handling (now handled by items directly)
* Made use of this.$$ to ensure tool works in other frames

ve.ui.Tool.js
* Made tools inherit from widget
* Moved trigger registry event handler to a method

ve.ui.Context.js
* Switched from using menu to contain toolbar to a simple wrapper

ve.ui.js
* Added get$$ method, a convenience function for binding jQuery
to a specific document context

ve.ui.*Widget.js
* Switched to using a config options object instead of individual arguments
* Added options
* Factored out flags and labels into their own classes
* Refactored value setting methods for inputs

ve.ui.MenuWidget.js, ve.ui.MenuItemWidget.js
* Broke items out into their own classes
* Redesigned API
* Updated code that uses these classes
* Added support for keyboard interaction
* Made items flash when selected (delaying the hiding of the menu for 200ms)

ve.ui.LinkTargetInputWidget.js, ve.ui.MWLinkTargetInputWidget
* Refactored annotation setting methods

Change-Id: I7769bd5a5b79f1ab36f258ef9f2be583ca503ce6