In some applications people might need to be able to pass more metadata
than the only "display-name" this commit allow that by adding a field in
all signalling structure where it is free form json and removing the
"display-name" as the free form `meta` field allows application to pass
this kind of information without any problem.
On the "webrtcsink" side this commit adds a `meta` property as a
gst::Structure which will be passed as a json string to the signalling
server so we enforce data to be structured.
The buttonMask variable was useless and using it as the button index
in the navigation events was nonsensical.
In addition, the button index must be increased by one to map it to
the X domain.
