htsp: add delayed unsubscription in response to HTSP request

The reason for this is there isn't a command for "changing" channel, this may
be changed in the future. So to simulate it the subscription weight is change
to a minimum level and then a new one can be created, thus reducing the need
to stop the tuner (especially for in mux zaps).
subscription: change some hardcoded to prioties to defines
webui: stop multi select combo selecting the wrong thing

this was because it reverted to using the display string and doing
a reverse mapping. Which is clearly the wrong way to do things when you
can have multiple display fields for the same key value.