Built-in help inaccessible with non-US keyboard

[FloydATC]

Describe the bug
The default key binding for Help (Ctrl+"?") does not work, presumably because "?" is typed with Shift+"+" on some non-US keyboards and neither Ctrl+Shift+"+" nor Ctrl+"+" is accepted to mean Ctrl+"?".

To Reproduce

1. Use non US keyboard layout
2. Hit Ctrl+"+" or Ctrl+Shift+"+" (which is where "?" is mapped)
3. Nothing happens because this is not Ctrl+"?"

Expected behavior
3. Get help

Desktop (please complete the following information):

• OS: Ubuntu Linux 20.04.2 LTS