use bootstrap `disabled` instead of `ui-state-disabled`

in menubar
Added relative import of RevealExporter to __init__.py inside exporters module.
Added another missing import inside transformer module.
Merge pull request #3520 from damianavila/import_reveal

Added relative import of RevealExporter to __init__.py inside exporters module
Merge pull request #3514 from minrk/ui-state-disabled

use bootstrap `disabled` instead of `ui-state-disabled`