add bold monospace font specs to canvas_vars/ardour3_ui_default.conf
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)