(Note: This commit also changes the UI)

Changed:
-------------------
void obs_load_sources(obs_data_array_t *sources_list);

To:
-------------------
void obs_load_sources(obs_data_array_t *sources_list,
		obs_source_load_cb callback, void *private_data);

Signals should really never be required to use to make some function
work properly.  The "source_load" signal was required for the
obs_load_sources function, but it's meant more for loading private data
in the settings, not for general loading of sources.

This changes it so that a callback is explicitly required to load the
sources.