Trim automation is planned via SlavableAC as normal AutomationMode.
Some of this code have a revival (a special "Trim+Preview" state
before merging Automation but that has to be more general than Pan & Gain.
The first click to select a new stripable may actually un-select strip.
counting and ignoring this leads to strange behavior.
Also when the selection_counter hit zero, VCAs were not ignored.
whatever this workaround was good for, it's more trouble than it's worth.
As side-effect this also improves DnD + Gtk::Entry behavior with stock GTK.
Remove old code to remove duplicate engine state which was used to
work-around a bug since 3.x saved duplicates but broke with 4.x separate
i/o devices.
Add a new variant to purge old unused states per backend (after sorting).
* add an abstract StripableTimeAxisView (Route TAV + VCA TAV)
* move common strip-methods into STAV
* Add Automation Lanes to VCA TAV
* Allow ATAV without Automatable for VCA Controls
Cloning a region retains the region-lock style.
Playlist partitioning set the region start-property only (audio-time).
If a MIDI region is locked to musical-time, Properties::start is ignored
and overwritten by Properties::start_beats.
* The UI and ctrl-surface controls use and display the combined value,
including control-masters.
* The Automation lane of a control is the raw value of the control
without masters.
When touching (or writing) automation, the control-master needs to be
factored out (or subtracted). e.g press+hold a control -> write inverse
master automation.
This went unnoticed because: VCA gain automation was always applied
(regardless of automation state) but when it was not playing
master_ratio() factored it out again (per block).
