plugin menu for embeded plugins now groups by module

Plugin lists in the frontend editing were not grouping by module.  This only happened in plugins within plugins, like with the Mutli Column and Grid Column plugins.  The 'main' plugin dropdown for that placeholder / stack was grouped correctly though.

Looking for feedback on this because I'm unsure if the modification I've made would allow plugins to show up in the list that shouldn't.  I don't think so, but not 100%