MIDI editor: Option to limit selection to active channel (for multi-channel editing)

When editing multi-channel tracks, it is often (almost always, actually) useful to be able to limit event selection to the active channel, in order to limit the actions of editing functions to only that channel. This is similar to how tracks can be set to active/editable/visible.

I have actually written a Lua script for this option, but as expected, it becomes impractically slow when there are tens of thousands of CCs in the track.

There are probably more than one way in which this can be implemented. One way would be to include a new option:

Option: Limit note and CC selection to active channel of active take.

Alternatively, as suggested by MikComposer, a toggle button to limit *all* CC editing actions to only the active channel. (Which can include selection.)

Existing alternatives include:

* Using the Event Filter window, in which the target channel can be set manually, together with "Invert", and to click "Remove from selection" after each selection. This not an intuitive, smooth alternative, however. Moreover, there appears to be a bug in the Event Filter: while "Invert" is selected, the active channel cannot be changed.

* The Event Filter can be activated to show only the active channel, but then the user loses all context of the other MIDI data in the track.

MIDI editor: Option to limit selection to active channel (for multi-channel editing)