(API Change) Use 'get' convention in libobs (base)

Instead of having functions like obs_signal_handler() that can fail to
properly specify their actual intent in the name (does it signal a
handler, or does it return a signal handler?), always prefix functions
that are meant to get information with 'get' to make its functionality
more explicit.

Previous names:             New names:
-----------------------------------------------------------
obs_audio                   obs_get_audio
obs_video                   obs_get_video
obs_signalhandler           obs_get_signal_handler
obs_prochandler             obs_get_proc_handler
obs_source_signalhandler    obs_source_get_signal_handler
obs_source_prochandler      obs_source_get_proc_handler
obs_output_signalhandler    obs_output_get_signal_handler
obs_output_prochandler      obs_output_get_proc_handler
obs_service_signalhandler   obs_service_get_signal_handler
obs_service_prochandler     obs_service_get_proc_handler