Ability to bind MIDI CC to tempo

[riban-bw]

Is your feature request related to a problem? Please describe.
It may be advantageous to be able to bind a MIDI CC to the zynthian's tempo control.

Describe the solution you'd like
Allow MIDI learn of tempo on the Tempo Settings page, similar to any other control.

Describe alternatives you've considered
Use onscreen controls.

Additional context
It would be consistent behaviour if the standard MIDI learn mechanism worked for controls on the and all other control views.

[riban-bw]

This is complicated by the fact that tempo view does not derive from control view. If it did, this would be fairly simple. @jofemodo should we refactor tempo to derive from control?