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.
Please add a docblock for this - especially to document what kind of information can be passed in as `options`.