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.
Removing one of the two ways from the ExclusiveOptionGroup is not a good idea since it is public API which other code my rely on.

But if we think only one option should be there to define the *selected* state this class should only offer that.