ve.ui.Context: Add embedding feature

Objectives:

* Make the context menu display in the top right corner of the currently
  focused inspectable node (if there is one)
* Prevent clicking on anything to do with the toolbar or popup from doing
  anything at all, ever

Bonus:

* While we are using the clever feature in jQuery's on method which allows
  passing boolean false to cancel the event - may as well do that in
  ve.ui.Dialog as well

Changes:

ve.ui.FocusableNode
* Add ability to specify the focusable element so that dimensions can be
  derived from it

ve.ce.Surface
* Add quotes to object keys

ve.ui.MediaDialog
* Change association from being MW specific to handling images in general

ve.ui.Context
* Add embedded styles for context
* Add embedded mode, which is triggered when the context is a single
  focusable node, and the node is large enough to fit the context
  reasonably

ve.ui.Dialog
* Inline mousedown handler

ve.ui.Toolbar, ve.ui.PopupWidget
* Cancel stray mousedown events

Change-Id: I4b25d33f64b4bcb8a3ecfd7e9728f54a2d4886f3
Merge "ve.ui.Context: Add embedding feature"