Add visual indication for events when the synth engine exceeds the currently available partials

[sergm]

As discussed in #103, there should be a new LED widget alongside the MIDI MESSAGE one that is shown:

• grey - when all partials are free
• green - when some partials are in use and there are enough free partials to play a new poly
• yellow - when an old poly is prematurely silenced to ensure free partials for playing a new poly
• red - when a new poly cannot be played because of insufficient free partials and the partial allocator cannot free some to fulfil a NoteOn request

The red indication should take precedence over the others. The red and yellow indication should be shown for at least 250 millis, and be clearly distinguishable from the regular green indication.

Requires #111.