Transpose tool will only transpose initial key signature …

[miiizen]

…on a system if there is a key

If the first key signature on a system is a change to the measure before, it will be transposed. Otherwise it will be left untouched.

Resolves: #18404

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[RomanPudashkin]

Ideally, we should make this function static and remove it from this class (perhaps declare it inside utils.h or directly in transpose.cpp). We are trying to make the implementation of engraving items as simple as possible (so they won't implicitly use systems/measures/etc)

[cbjeukendrup]

I pushed some code review fixes (some unnecessary includes); if you're happy with those, I think it can be merged.

[miiizen]

@cbjeukendrup looks good to me!