Linux: cefclient: Add a GTK implementation of CefJSDialogHandler (issue #1258).

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk/cef3@1760 5089003a-bbd8-11dd-ad1f-f1f9622dbc98
- Linux: cefclient: Add a GTK implementation of CefDialogHandler (issue #1258).
- Add new CefGetExtensionsForMimeType function.

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk/cef3@1761 5089003a-bbd8-11dd-ad1f-f1f9622dbc98