MidiInterface: create virtual ports more properly

Midi does not own an RtMidiIn anymore, it will use the first available interface (ie. "Virtual MIDI In" from Minotor)
Minotor does not crash (segfault) on close anymore (_rtMidi was deleted twice)
ConfigDialog: new "General" tab with renderer size

Add a new tab to ConfigDialog;
Add controls to set renderer size (height, width)
Implements accessors in Minotor
Add notice for user: Minotor need to be restarted when size is changed