adjust meter-width when configuration changes

fixes http://tracker.ardour.org/view.php?id=5592
update processor-name (short|long) when mixer-strip width changes
Wrap calls to add_shortcut_folder() in try { ... } block.

Gtk::FileChooserButton::add_shortcut_folder() throws a Glib::Error
exception if the folder being added already has a shortcut, causing the
'New session' dialogue to appear empty when choosing 'New...' from the
Session menu. This happens if Ardour's default session folder is
set to a location that has a standard shortcut in the Gtk file chooser,
e.g. the user's home directory, desktop, &c.

Catch the exception in ArdourStartup::setup_new_session_page(), so that the
'New session' dialogue appears as expected, and likewise in
ARDOUR_UI::open_session() to avoid a similar problem there.

Fixes #5591.
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.
use fixed fall-off time for k-meter
clean up video related messages
implement meter-lineup and VU standard preferences
BBC meter labels are always white
clarify meter preference options
change default VU mode to "0VU = +4dBu (standard)"
add "0VU = +8dBu" VU meter config
limit width and resize meterbride window
meterbridge: meter-type dependent color-schema