Support loading stylesheets into frames from different locations

This is one of the blockers for splitting VE up into separate
repositories or extending VE with an extension.

ve.ui.Frame.js

  It's critical that we don't emit initialize from ve.ui.Frame until
  it's completely loaded, especially its styles, because we will
  begin measuring it straight away.

  Involved loading the stylesheets using $.ajax and setting base
  URL of the iframe to the ve.ui styles directory so all the image
  URLs still worked. This won't work for stylesheets from multiple
  locations, so we needed a more robust solution.

  The new solution uses some trickery described in the code
  documentation, but essentially no longer depends on all
  stylesheets being located in the same folder.

ve.ui.Dialog.js, ve.ui.Inspector.js, ve.ui.Window.js

  Static methods are now being used to extend a window class to
  include different stylesheets rather than simple array
  concatenation.

Change-Id: I619238732f975d41305f81f8f818a577a40f49da
Merge "Support loading stylesheets into frames from different locations"