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.
The goal was to follow the methodology of the ExclusiveOptionsGroup as closely as possible for consistency but add the checkbox group to provide non-exclusive options.  I think for consistency either both should have the function or the function should be removed for the ExclusiveOptionGroup.