Allow playing on demand items from the PVR Guide

[michaelarnauts]

This PR allows the user to play Video On Demand content from the Kodi PVR Guide.

This requires the channel json to include a vod field, with an URI that we can use with PlayMedia to play a program based on the aired date.

This is the only supported way in Kodi Leia, for Matrix, more fancy stuff is possible, but that's for later.

This is implemented in the following PR's:

• https://github.com/add-ons/plugin.video.vtm.go/pull/181
• https://github.com/add-ons/plugin.video.vrt.nu/pull/760
• https://github.com/add-ons/plugin.video.viervijfzes/pull/29

Partially implements #1

[michaelarnauts]

I think this is okay for now. I will probably have to hide the context menu on Matrix, but that depends on how nice we can integrate there.