fix #275483: Added the ability to change the color of the playbackCursor

[SKefalidis]

Resolves: https://musescore.org/en/node/275483

It gives the user the ability to change the color of the playback cursor (can't be made fully opaque).
Because having a fully opaque cursor does not have an obvious use if you choose a fully opaque color you get the default transparency level, so the user can change color with even fewer clicks.

To solve the original issue you make the cursor transparent and the voice color black.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• [n/a] I created the test (mtest, vtest, script test) to verify the changes I made

[igorkorsukov]

Better name: MAX_CURSOR_ALPHA, to immediately see that it is a constant

[SKefalidis]

@igorkorsukov I chose lower-case because the other constant in the same file is like that. Should I change both?

[igorkorsukov]

Only this one is better. Others are best fixed with a separate pull request.

[Spire42]

I already have this implemented as part of my redesign of the playhead (some discussion here) in collaboration with @Tantacrul.

My work is currently in progress so there's no PR yet.

[SKefalidis]

@Spire42 Should I close this then?

[Spire42]

@Spire42 Then I should close this, right?

Yes, I think that would probably be best. Sorry about that.

[SKefalidis]

Alright then :^)