20998: Added a midi mapping for "Enter rest"

[krasko78]

Resolves: #20998

Adds a midi mapping for the "Enter rest" ui action which is also tied to the "Enter rest" shortcut (normally 0 and Num 0).

I think I am seeing a couple of issues with the "Enter rest" action in general:

• In Rhythm only input mode, "Enter rest" acts as "Toggle rest". Is this expected? Does "Enter rest" make any sense in this mode at all?
• More importantly, in Re-pitch only mode "Enter rest" enters a rest but changes the duration to the currently selected duration instead of retaining the whatever duration the underlying note has (similar to "Input by note name" mode). I don't think this is correct. Personally I enter my scores in Rhythm only mode, then Re-pitch only mode. During the first phase, I'd enter a rest as a note being interested only in its duration, and then during the second phase I'd change the note to a rest but I'd not expect the duration to change.

Thoughts?

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