Version 7.1 fixes.

Update changelog and documentation.
Add new SkyboxSwapper plugin.
Added modified version of the unity built-in Skybox/Panorama shader to support the SkyboxSwapper plugin.
Fix the multi-subscriber Unity player to use the _MainTex property instead _Emission.
Rename prefab BasicQueue to GeneralQueue.
Rename script SimplePlaylist to Playlist.
Rename prefab SimplePlaylist to AncientPlaylist.
Update demo scene floor to be seethrough.
Remove unused playing threshold config option from TVManagerV2 as it's no longer used.
Add configurable auto resync interval value.
Fix improper implementation of the ChangeSeek methods to support correct usage of both percent and exact time seek.