Add file_browser_callback_enabled setting to enable file browser icon regarding filetype

See http://www.tinymce.com/develop/bugtracker_view.php?id=6047

A new option file_browser_callback_enabled is created to determine if the browser icon should be shown or not.

For now, the filetype possible are : 
- file (used in link panel)
- image (used in image panel and poster in video panel)
- media (used in 2 times in media panel)

the file_browser_callback_enabled settings could be set to : 
- undefined (default) or true : enabled file_browser_callback if provided everywhere
- false : disable file_browser_callback everywhere
- string : enable file_browser_callback when the filetype is containing in the string.


Exemple : in order to allow only image and file browser : 
- Set file_browser_callback like before
- set file_browser_callback_enabled to "file|image"

I didn't find any place to add automatic documentation for this new setting. Did I miss somthing?