Fix #275483: Give [playback cursor] a distinct color preference

[worldwideweary]

Resolves: https://musescore.org/en/node/275483
Reopens the closed PR: #5848

• Makes a slight adjustment to the previous code using a hex value rather than string to be in conformity with the surrounding code as it now currently is.

Description: Adds a distinct color for the playback cursor: defaults to 50 alpha level if user attempts full opacity. And, as mentioned in the original PR, 100% transparency is allowed to have no cursor showing upon playback.

P.S. I consider this a bug-fix in the sense that there was a previous closed issue for specifically changing the highlight color as if it was implemented totally, but in reality it bound "Voice-1" color with "Playback cursor" color. As such, their separation could be seen as a bug fix. Point of mentioning this is that this should, in my opinion, be included into 3.6.3 if released and not sat around only for version 4 later, especially since this was available months back (the closed PR mentioned above) but not included due to a false alarm of someone else stating that they would be providing something better which never came to pass.

• 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
• I created the test (mtest, vtest, script test) to verify the changes I made

[Jojo-Schmitz]

I wonder whether this would need to get ported to master