Set default MIDI output channel in Preferences.

[theGreatWhiteShark]

Currently the Channel set in the Preferences Dialog corresponds to the one used for all incoming MIDI messages. The channel for outgoing message is set in the Instrument Editor for each instrument separately.

Now, when the user attempts to setup MIDI output she sets a MIDI output port/device in Preferences > MIDI System and probably expects triggered notes to be send as MIDI messages as well. But they are not. Because if the channel in the Instrument Editor is set to off (-1 within the drumkit.xml) no MIDI messages will be generated. So, to properly setup MIDI output, a channel has to be assigned for each and every instrument manually.

But this is not an edge-case for kits which do not have the MIDI output channel set properly. In all drumkits I have at hand (and these are a lot) only "rumpf_kit_z01_gm_freshy" sets an output channel for a single instrument. All other instruments have a channel of -1.

As this only leads to confusion I would suggest break this long standing UX in favor for a more intuitive one:

1. We provide the option "Default Output Channel" in Preferences > MIDI System which will be used per default for all outgoing MIDI messages. Setting a channel of an Instrument in the Instrument Editor will overwrite the default channel.
2. Instead of using the check for a MIDI output channel of a note != -1 to determine whether a MIDI message should be send, the presence of a MIDI output port/device in the Preferences will do. This states way more clear the intention of using MIDI output and enabling/disabling MIDI output can be done way more easily

As a consequence we loose the ability disable MIDI output for specific instruments. But I don't think this will be an issue since the ability to customize both channel and pitch of each instrument gives 16*128 different combinations. One of them can surely be sacrificed for discarding all incoming notes.

@cme , @trebmuh what do you think?