Copy-paste bug fix

[saintmatthieu]

Resolves: #5523
Resolves: #5599

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• The title of the pull request describes an issue it addresses
• If changes are extensive, then there is a sequence of easily reviewable commits
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Recommended:

• Each commit compiles and runs on my machine without known undesirable changes of behavior

QA:
This PR intends to restore the exact same behaviour as that of 3.3.3 insofar as the "Always paste audio as new clips" option is disabled and no stretching is involved.

I put together this truth table:

Always paste audio as new clips	Editing a clip can move other clips	One clip on clipboard only	Position	Side effect on existing clip
OFF	OFF	False	Before	not-enough-room error
OFF	OFF	False	Beginning	not-enough-room error
OFF	OFF	False	Middle	not-enough-room error
OFF	OFF	True	Before	not-enough-room error
OFF	OFF	True	Beginning	Shifts data to right and merges
OFF	OFF	True	Middle	Shifts data to right and merges
OFF	ON	False	Before	Shifts data to right
OFF	ON	False	Beginning	Shifts data to right
OFF	ON	False	Middle	Shifts data to right
OFF	ON	True	Before	Shifts data to right
OFF	ON	True	Beginning	Shifts data to right
OFF	ON	True	Middle	Shifts data to right and merges
ON	OFF	False	Before	not-enough-room error
ON	OFF	False	Beginning	not-enough-room error
ON	OFF	False	Middle	not-enough-room error
ON	OFF	True	Before	not-enough-room error
ON	OFF	True	Beginning	not-enough-room error
ON	OFF	True	Middle	not-enough-room error
ON	ON	False	Before	Shifts data to right
ON	ON	False	Beginning	Shifts data to right
ON	ON	False	Middle	Splits and moves to right
ON	ON	True	Before	Shifts data to right
ON	ON	True	Beginning	Shifts data to right
ON	ON	True	Middle	Splits and moves to right

There's a lot of redundance I didn't bother cleaning up, sorry about that.

Explanation of column names:

• "One clip on clipboard only": the region you selected before copying only contained one clip. It is important because pasting more than one clip should prohibit merging.
• "Position": the position of the cursor (cursor-paste) or the end of the selection (selection-paste):
• • "Before": strictly before the beginning of a clip
• • "Beginning": exactly at beginning of a clip
• • "Middle": within a clip

Besides that:

• Scenarios of that table relevant to selection-paste (as opposed to cursor-paste) also work
• Applying Noise Reduction doesn't create new boundaries (if unstretched)
• Nyquist effects that modify length (e.g. Delay) create new boundaries
• Nyquist effects that don't modify length (e.g. Tremolo) on unstretched clip don't create new boundaries

[vsverchinsky]

I wonder what if both are true? I inspected calls to that function and didn't find place where it's the case.

[saintmatthieu]

I don't think either that both happen to be true. AFAICS if split is true, whether clearByTrimming is true or not doesn't make a difference. If the former is false, though, we clear by trimming (as opposed to completely deleting) but the resulting smart clips on the right are moved back left.