static global definition for track-header width

the track-header minimum width is defined by
the name-label (IFF the 2nd row fader is not visible,
but the fader is rather small by default and and grows)

track-header width in almost all cases is:
   name-label width
 + width of three button (1 char each)
 + max size of all meters (if visible)
 + width of MIDI scroomer (if visible)
 + 2px table cellspacing (per column)
ArdourButton: prepare for [em] based padding
number-label: use pixel-width for given font
compact meter+fader for > 6 channel tracks