Add WebSocket event when video is played

**Is your feature request related to a problem? Please describe.**
The new "Videos" feature within the Video Switch tab has the following warning text:
> Videos are not necessarily chroma-key safe. Ensure you are displaying the scoring display without chroma keying (transparent overlay is ok) when playing videos or play videos through an external system.

**Describe the solution you'd like**
I'd like a distinct event emitted over the Match Stream WebSocket whenever a video is started. Automated video switcher scripts/OBS plugins can then consume this event to switch to a distinct scene with no chroma-key applied.