reset meters only when *really* necessary

..and continue to calculate fall-off in
audio-cycle (rather than UI thread)

TODO: check if this works properly when switching
between audio/midi meter modes on a midi-track.

One of the motivations to always reset meters when the
meter-point changes was to resolve peak-hold & fall-off
issues when a midi-meter replaces an audio-meter and vice
versa.
Don't allow to deactivate meter.

before this change:

1) switch to 'custom' meter point,
2) deactivate meter processor.

-> meters does not run regardless of meter-point
-> meter can only be re-nabled in 'custom' mode
tweak label alignment
VU meter label color&shadow tweak by the_CLA
set midi-metric ticks depending on meter-type
prepare horizontal meters (level-meter H/V Box abstraction)
vtl: replace Ardour with Program-Name in messages
[re]implement horizontal meter