fix #16077 part 1: Double click an instrument name to open the corresponding menu

[SKefalidis]

Resolves: https://musescore.org/en/node/16077

When the double-click event is created, the program checks if the user double-clicked on an instrument-name and opens the 'Edit Staff/Part Properties' menu if that is the case.

This provides one part of the requested functionality (the other, is to add the same ability for footers, but this should be another PR).

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[Jojo-Schmitz]

Opening the staff properties dialog is not exactly the selected as allowing to edit the instrument name in place though.
As that had been disabled back in the day as it caused issues, doing it this way now seems the safer way though

[SKefalidis]

From what I understand the user wants the ability to double click to open the corresponding menus in both cases. He moves on to say that it would be ideal if he could edit-in-place but this is not the main focus of the issue (as its title suggests).

Giving the ability to edit in place would be truly ideal, but it would probably require reworking the entire Instrument-system-iname chain of interactions.

[Jojo-Schmitz]

No objection from me, just pointing out the shortcomings of this solution.

[SKefalidis]

Fixed formatting and added a return statement.

[MarcSabatella]

I like the idea a lot. Any possibility the same could be done for header/footer? I'm not as sure it's possible, at least, not as simply - I kind of doubt the header/footer register here. But worth a shot.

[SKefalidis]

@MarcSabatella I am actually finishing the PR for that right now. :-)

[igorkorsukov]

I think there is no need for else

if (!clickedElement)
    return;

if (!clickedElement->isEditable()) {
    ...
    return
}

...

[SKefalidis]

I made the changes which were suggested.