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.
If this class should be used generically wouldn't it be good if it would either expose some of its variables as public members or provide functions to get more information about added buttons and also a function to add more?