Add Select Fixture as option for Push Fader or Encoder Push#136

It would be very nice to add a feature to set SelFix to the Push Fader or Encoder Push from the fader layout option. Use of SelFix as selection of the fixtures involved in the relative playback.

We can possibly do so in one of the next feature updates, depending on customer feedback and the priorities we will discuss once we start planning the update.