Refactored filebrowser-popup.js to use iframe instead of directly loading one document into another
Further refactoring of filebrowser-popup.js

* Now popup is namespaced under mediaLibrary.
* Only initialisation happens on document ready, while opener function is available in the global context.
* Callback execution is now asynchronous to cater for certain editors.
* Added a compatibility alias so that existing libraries can keep using it.
Reduced filebrowser popup size, removed dialog padding, fixed delete table header
Clean-up the events in filebrowser-popup.js
Version bump to 0.2.28