Add support for NDI cameras

[cimnine]

This provides an implementation to control NDI-enabled PTZ cameras with obs-ptz.

Some code is copied and adapted from DistroAV, which is licensed under GPL-2.0 as well. I've added copyright headers where applicable (ndi-finder.* and ndi.*).

Please review this PR carefully. I'm not experienced at all in C++/C, so I might have made grave mistakes. Also, I need help figuring out the cause of the Known Issues I mention below.

Related issue: #56

Implemented

• Initializing of the NDI library
• Finding NDI cameras on the LAN
• Controlling NDI camers (PTZ, Home, Presets store & recall)

Known Issues

• The finder_callback does not have the desired effect (cb is invoked, but modifications to dropdown don't appear in UI)
• source_name is somehow not persisted across OBS restarts
• Presets are not persisted either

Not planned

• Only show NDI cameras that actually support PTZ. This would require connecting to each discovered camera and checking whether it supports PTZ.