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 it's a directory or a 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 to use magickcore it will require implementing some
extra cmake options.
Add image source plugin

This plugin will use image file support implemented by the core in order
to display images files as a source.
Add image source module to OBS

...We really need to make this automatically detect the modules via path
and load them.