Add submodule to integrate with selective_better_exposed_filters

Problem/Motivation

Selective Better Exposed Filters → allows you to limit the options in your widget to only ones with results in the view, but only works with standard widgets

Proposed resolution

Create optional submodule for this functionality on a similar widget to the existing multiselect_dropdown_bef that will add the options offered by Selective Better Exposed Filters

Remaining tasks

• Review of approach
• Code review/cleanup
• Tests