minor refactoring, improvements, etc.

Some minor refactoring and other small design improvements, including
moving 'dispatch_next_event' routine from MIDI_EventFilter to its parent,
MIDI_StateMachine; and adding a 'filter' (MIDI_EventFilter object accessor)
attribute to AppConfiguration and using it in MIDI_EventStream instead of
the previous design of MIDI_EventStream creating the MIDI_EventFilter
object.  And removed a bit of old code, such as checking for the
BANK_SELECT state and the PROGRAM_CHANGE state where they will never
happen.
Transposition logic bug fix

Bug fix: When two or more transposition specs overlap such that a particular
pitch is affected by all of these specs but only one of the specs has been
activated (i.e., the user has pressed a pedal/key combination activating
only that spec), one of the other overlapping specs was often applied,
incorrectly, during a transposition.  The problem was that the logic
identifying which spec was active (i.e., had been toggled "on" via
toggle_transposition) was missing - the notes affected by the transposition
were marked as "to be transposed" but the first transposition spec found
that contained the current pitch would be used during the actual
transposition instead of the correct one.  The fix was to change
toggle_transposition to associate each pitch within the range to be
transposed (instead of, as before, just marking the pitch with a boolean
status - to-be-transposed) with the pitch_value argument to
toggle_transposition (i.e., the value used as a key to id a specific
transposition spec [e.g., in: 'transpose_spec: 68 48..71 +7', the 68 is such
a key]) so that the key to the right transposition was available.  Then, in
'dispatch', retrieve that key (stored in the transposition_status array with
the current pitch as the index) and use it to retrieve the correct
transposition spec.
_set_state -> set_state; docs; config[12], etc.

Various small changes: In MIDI_StateMachine _set_state was changed to
set_state to indicate it is public, since it is called by other classes.
Made this change in the code that calls _set_state.  Removed some
debug/print statements in Regular_MIDI_Event that were inserted while
working on the transposition bug fix.  Filled in some more documentation
(README.md).  Tuned config1 a bit and added a new file, config2, which
sends to a2j_bridge, which on the JACK MIDI side is to be hooked up to
ardour - thus allowing midifilter to output to ardour (3).