UiMidiInterface: silent debug message when mapping is empty
Midi: new function findMidiInterface()

This function takes an id and returns corresponding MidiInterface pointer, NULL otherwise
MidiInterface: improve sendMessage() function

It now takes a new argument: channel.
Implementation uses defines instead of hardcoded decimal values.
Midi: add feedback to MinoTrigger

 - MinoTrigger does now have a feedback(bool) signal to propagage associated boolean changes (default: toogle's status of trigger itself);
 - MidiMapper does handles triggers feedbacks and send a MIDI message to toogle corresponding LED on Korg nanoKontrol 2;
Core: enable new feedback feature for Play and Stop feedback

 - 'Stop' uses default feedback (ie. trigger's status), which illuminates 'Stop' button when user press it.
 - 'Play' uses a custom signal (ie. clock source status) which illuminates 'Play' button when clock source is running (internal or external).
MidiMapper: constify some functions and/or arguments
Fix typo "toogle" -> "toggle"
Midi: change internal API of MinoTrigger and MidiMapper interaction
MidiMapper: code clean up in MidiMapper::triggerFeedback() function
MidiMapper: fix signals disconnection
MinoClockSource: enable "Tap" button blinking on tempo
MinoClockSource: move beatToggled() signal from Minotor to MinoClockSource

 - Some code clean up
 - Fix beat status when stopped