Use ve.ui.SurfaceWidget for image captions

Objectives:

* Make use of the new ve.ui.SurfaceWidget.js for image caption editing

Changes:

ve.ui.MWDialog.css
* Remove styles that are no longer needed
** -body class isn't needed if you contain the content in a layout and
   configure it to be padded
** Toolbar styles are now part of surface widget

ve.ui.MWMediaEditDialog.js
* Wrap caption fieldset in an editPanel (also nice because we may merge
  this dialog with MediaInsert and use a stack panel layout in the future
* Replace surface and toolbar with surface widget which manages both

Change-Id: I1b7e0e2ec1cfa30879360a7ebde2b0f35d26aca5
Merge "Use ve.ui.SurfaceWidget for image captions"