Toolbar action widgetization and UI refactoring

Objectives:

* Use widgets to render toolbar actions
* Remove labels next to help notices and edit notices buttons
* Add a close button to the help notices and edit notices

Overview:

* ve.ui.ButtonWidget is now abstract, use ve.ui.PushButtonWidget instead
* ve.ui.IconButtonWidget now inherits from ve.ui.ButtonWidget
* ve.ui.PopupWidget's display method no longer takes x and y arguments
* Fixup naming issues in MWCategoryPopupWidget
* Fixup naming issues with some ve-init-mw CSS classes
* Rename ve-mw/ui/styles/ve.ui.Widget.css to ve.ui.MWWidget.css
* Change uses of "callout" to "tail"
* Add hyperlink functionality to buttons
* Make buttons accessible through focusing, but make unfocusable by
  clicking
* Add head option to popup for rendering a title and close button

Bug: 52386
Change-Id: Iea2c8df1be64d40f9c039873d89ee540cc56e687
Merge "Toolbar action widgetization and UI refactoring"