Accessibility: Note input overrides menu navigation shortcut

Note input (keys a through g) overrides features of menu navigation.

For accessibility reasons the Qt menu provides some keyboard navigation features:
When pressing Alt and an underlined letter in the menu bar, the entry opens. It is then possible to navigate the options using Tab, Shift+Tab, Enter and Escape, or using the arrow keys.

Some entries provide additional navigation options: Subentries with underlined letters can be selected by pressing the corresponding key.

When this menu navigation technique is tried (while the menu is opened) using a letter between A through G, instead of selecting the subentry the note corresponding to the letter is inserted (or when not in note input mode the pitch of the active note is changed). This prevents users from using the navigation option.

Expected behaviour:
When a menu entry is selected, single letter shortcuts are disabled.

Which subentries are affected depends on MuseScore's display language. To reproduce the bug in English locale compare e.g. Alt+A then M to Alt+A then F.
The problem is very minor because there are 2 alternative navigation options.