Add obs_source_previewing method

[alessionossa]

Currently there is now way, afaik, to know if a source is "In Preview" when using Studio Mode.
There are already obs_source_showing and obs_source_active methods, but obs_source_showing always returns true (and is expected to do so) when using Multiview.
The method can be useful to send Tally Light preview's signals.

Example of usage and issue with the current implementation:
https://github.com/Palakis/obs-ndi/blob/adcf4f09acd02cf355c7f9a2541232adbb480d1a/src/obs-ndi-source.cpp#L495
obs-ndi: NDI Preview Tally sent if Multiview is opened. #318

[alessionossa]

UPDATE
It will probably be implemented soon
DistroAV/DistroAV#609 (comment)