Add support for mp4, webm, and ogg

[pianissi]

For #466, adds support for automatic and on hover playback for videos in gallery view. Also allows playback in main view, which has the side effect of allowing sound from non-video .ogg files. Implementation uses web video elements which replace the image elements used normally. I'm unfamiliar with React or Electron so the implementation may not be optimal.

Caveats:

• No thumbnail images
  • As a result video preview will always be on the first frame
• On gallery view, automatic playback of many videos lowers performance by a noticeable amount

[Vargock]

Sincerely hope that it will be implemented into the main branch sooner or later. With some video support, Allusion would be pretty much feature complete, at least for a user like me.