Make menu shortcuts appear more decent

[toaster]

@andydotxyz had an idea regarding the appearance of (non-native) menu shortcuts (#2715 (comment)):

the menu shortcut text is as clear as the menu text (and disabled appropriately) but should it be more subtle? (I think I have seen some places do that).
Would having the shortcut hints be in the disabled colour too light? do we need a "caption" colour that would make sense or similar?

I think the idea is worth to be discussed and that mock-ups should be created to show the visual effect.

[matwachich]

Another thing about the shortcut text format.
I don't think that ^A or !A or +A id understandable by common users, especially on Windows, where it is more common to see Ctrl+A or Alt+A or Shift+A

[andydotxyz]

Fixed on develop :)