automatically set ArdourDropdown box width.

..according to longest text in English.
shorten grid/snap label "Timecode" -> "TC"
reduce space around TearOff Tickmarks
Revert "ardour loves narrow screens part one"

This reverts commit d3d83eac9df8f7c56fd6fedcf001dffa52d7357a.
restore full-height of editor-mixer.