Allow non-CMB2 pages to be added to options-page tab-groups.

1. A filter hook inside get_tab_group_tabs() could facilitate this
   $tabs = apply_filters( 'cmb2/tab_group', $tabs, $tab_group );

2. Additionally it would be nice if CMB2 would also provide a method to print out these tabs on the non-CMB2 pages. Even with the filter hook, the non-CMB2 page has to render its own tabs, replicating and slightly modifying code from get_tab_group_tabs() and options_page_output().

The second part would require a more involved solution. Separating the tab-printing logic from options_page_output() and making it available statically/globally to other plugins.
And there are probably other considerations to be thought through. So perhaps item 2 can be considered separately from the filter hook.