added QgsMessageBar::SUCCESS

Added a new level of messages in QgsMessageBar for succesful messages. they are colored with the bootstrap success green (background #dff0d8; border #8e998a) reusing the mActionCheckQgisVersion.png icon.
@NathanW2 can you have a look? thanks
Could you copy the icon and make a new one called `mIconSuccess`.  Reusing is good but it would be nice to be able to change the success icon without changing the other.