Improve screen reader readout of 'Voice assignment' buttons in properties

Follow up from https://github.com/musescore/MuseScore/pull/31853#issuecomment-3785572441 (pasted below)

Problem

Generally, when a radio button option that's part of a properties control is focused on by the screen reader, what's read is the property name followed by the button name, like this:

Position Above, radio button

...but for the 'Voice assignment' buttons, the property name is not read before the button names:

Task

Make the screenreader read out the following for each button:

All:

• For multi-staff instruments, when the 'parent' button is hovered, ideally it would read the name of whichever of the two options is currently selected:
  • Voice assignment, All voices on instrument, button
  • Voice assignment, All voices on this staff only, button
• For single-staff instruments, it should just read:
  • Voice assignment, All voices on instrument, radio button
    • if we can't make it say 'radio button' (as opposed to 'button' as it says now), that's okay for now.

Individual voices:

• Voice assignment, Voice 1, radio button
• ...
• Voice assignment, Voice 4, radio button