Added global hotkey to mute all users but the Presenter.

bigbluebutton-client/locale/en_US/bbbResources.properties

bigbluebutton-client/src/BigBlueButton.mxml

...client/src/org/bigbluebutton/main/events/ShortcutEvent.as

.../src/org/bigbluebutton/main/views/ShortcutHelpWindow.mxml

...rc/org/bigbluebutton/modules/users/views/UsersWindow.mxml