Allow manual setting of MIDI control change numbers (#8)
* Allow manual setting of MIDI control change numbers
* Changes controls for MIDI controllers in channel new / edit dialog to
`Gtk.SpinButton`s with range (-1, 127).
* Adds tooltip to controls.
* Changes `Channel` object in C extension to add methods for auto-assigning
the CC # for each control (volume, balance, mute, solo) individually.
* When editing channel, channel is only renamed when name actually
changes.
* Fix updating of spinbutton in MIDI learn callback
Signed-off-by: Christopher Arndt <chris@chrisarndt.de>