Don't fail jackd command line creation for jack dummy backend.

The dummy jackd backend doesn't require a device to be specified, so much
of the error checking in get_jack_command_line_string() is irrelevant, if
not actively wrong, when the dummy backend is specified.

Only perform the checks if the chosen jack backend is not the dummy.
Fix setting playback & capture channel counts for jackd dummy backend.

Unlike all the other jack backends which allow setting the number of inputs and
outputs with -i & -o, the dummy backend uses -P & -C for this. Make the jackd
command line use these options when the dummy backend is requested with a
specified input or output channel count.
Don't ever pass -d for device name to dummy jackd driver
Report an error to stderr if creation of jack command line fails.

Output a simple message to stderr if get_jack_command_line_string() still
fails for any reason.
fix MMC

MTC and MIDI Clock port input handling was moved into the process/RT thread(s) during audioengine work, but MMC was
left orphaned. Add it to the port(s) handled by the MIDI UI thread.

Also, remove PortChange request from MidiUI because it has no meaning anymore
fix placement of black/white note lines/bars in MIDI streamview
remove unnecessary initialization of fill color for streamview rect
fix computation of canvas text item bounding box
fix computation of rectangle bounding box
fix drawing of a canvas LineSet object
expand bounding box of all objects by 0.5 before computing intersection for render cycle; alter debug output
add Item::redraw() convenience method
redraw entire MidiStreamView after scroomer drag so that everything is redrawn
Use add_route_dialog's transient_for() window to determine track selection for order_hint.
Merge branch 'ordering_fixup' of https://github.com/nmains/ardour
Add a double-click handler for items in the editor window.
Make double-click on marker pop up rename dialogue.
Mark session dirty when a marker is renamed.
Double-click on region pops up region properties.
Edit tempo & meter markers on double-click.