various fixes for places that used get_font_for_style() ... converted to use UIConfiguration::get_canvasvar_*Font()
modifications to region drag implementation

(1) if we're dragging over the drop zone, then x-axis motion is irrelevant for threshold-of-move
(2) store original time axis view of a dragged region so that if we create a new track with the region drag, it can be the same height
change the way font specifications are used in UI config files

If no font family is specified, enforce use of Sans to match GTK behaviour (which we inadvertently relied on, it appears)
fix non-working cmd-Q on OS X. For some reason the shenanigans related to removing it from various menus as part of OS X standards breaks the accel system
fix port-unregister (on failed latency measurement)
rework raw-midi drain strategy (workaround for sync devices)
engine-dialog: properly restore previous active state
latency measurement dialog updates:

* distinguish errors: no ports and no engine.
* fix "No measurement results yet" message at successful end
* stop update callback
no more stack smashing, renice debug messages
correct conversion of drop y-position into trackview for drag-n-drop of external files
explanatory comment
ensure that all waveviews have ALL their colors set accurately before first render

Avoids occasional bright red zero line drawn because the color(s) had not been reset correctly
fix some confusion when redrawing editor/mixer track/strip displays after track/bus deletion

The code relied on the idea that the order-key resync that occurs after deletion would change the order keys and thus cause
a redisplay. But since both the editor and mixer can initiate an order-key resync, the other window's resync will actually
do nothing (the order keys will already be correct). This led to the incorrect placement of material in the tracks canvas,
because the first resync triggered a redisplay while the route still existed, and then the second resync didn't cause a
redisplay (repositioning) but the canvas elements representing the track went away.

Fixed by forcing a redisplay in both editor and mixer if a route deletion is believed to be triggering a row deletion
in their underlying data models.
alsa sequencer device/port listing
prepare ALSA sequencer
virtual abstraction of Alsa Raw+Seq
refactor Canvas so that all Items have children; add Container abstract base class; rename Group as "Layout" and retain only drawing semantics
use newly factored canvas in gtk2_ardour
Rename waves backend to match Tim's name
Rename jack backend to match Tim's name