re-nice rendering of channel-splitting icon
Revert "Revert new meter types (postponed until after 3.3 release)"

This reverts commit d80f672e8487f459d76ab291958bffcded08f0fd.
implement new meter types
minor tweaks to meter-names and legend
restore session backwards compatibility to 3.3
meterbridge metrics update:

* display metrics for every used meter-type
* show metrics between meters with differnet types
* update text and label alignment for meterbridge
* fix crash when deleting metrics-strip (zero route)
* clean up metric pattern code (use float for dBFS)
fix #5609 (import with 1 track per channel mis-associates tracks + channels)

when using ImportDistinctChannels, correctly name regions so that playlists for each channel/track
are also named differently, and thus reloaded properly when the session is reloaded.
Fix special handling of 'zoom vertical' scroll wheel modifier key.

gtkmm2ext/keyboard.cc has a special case to emit a signal on the key-up of
the modifier key used to adjust track heights in conjunction with the
scroll wheel, so that the same track continues to be resized even when
it's shrunk to no longer be under the mouse cursor. However, this code
assumed that the modifier key for this was <Shift>.

Fix it to use the event->state bit corresponding to
ScrollZoomVerticalModifier instead, and rename the relevant functions to
clarify that it's the 'zoom vertical' modifier key they're dealing with.

Partially fixes #5610.
Merge branch 'master' into clang