Add checkbox group for settings dialogs

This adds the class CheckBoxGroup to qt_gui_py_common for use in the SimpleSettingsDialog class.  Options are not exclusive and return an array of selected indexes and selected options when polled using the get_settings function.  

This class will be used to add optional plotting markers to rqt_plot and the rqt_bag plotting plugin.
I am ok with having the function even if I don't see any value in it.

Externally provided  groups can't provide a similar convenience functions and it feels like trivial duplication of API.

Anyway we can't remove the existing function for the ExclusiveOptionsGroup since it is public API. We can only decide if we want to add similar functions in the way. Either way I don't mind.