Add recording button

[sieren]

Here's an alternative take on #124 with a little less code and reusing the MIDIToggleButton.

Dev Changes
We're using a new class called MIDIRecordButton that inherits from MIDIToggleButton.
Instead of using PaintCode we add a Circle Subview and when isOn is true, we animate
the backgroundColor of said Circle View.

User Facing Changes
The UI is now more in line with the rest of Synth One. An active recording is indicated by both the time progressed as well as the animated button.

@marcussatellite @analogcode adding the SubView in layoutSubviews is probably not the right place, let me know if you know of a better function for this.