libobs:  Fill in rest of 'path' property code

A path should have a filter, default search directory, and a way to
indicate whether or not a directory or file is desired.
Implement 'path' property in to properties view
Add image file loading support to the core

Both ffmpeg and magickcore library support have been added and are
working, though the use of magickcore instead of FFmpeg will require
extra cmake options.
Add image source plugin

A simple plugin that does nothing more than display an image file as a
source.
Add image source module to obs-studio basic UI

...We really need to make this search for modules and automatically load
them.