There is really no need to make back and forth between a string and
int/enum representations, and it actually cause problems at times.
It's also a problem for the button representation where a modifier will
be represented as a key.
When no accelerator was set yet, display relevant information, and when
the button is toggled (waiting for shortcut input), display also
relevant info.
A widget to grab a shortcut provided interactively. It can also grab
modifier-only shortcuts so we will be able to use it for the new
on-canvas interaction customizability.
Maybe we'll use this later for the shortcuts dialog which should really
be improved some day and has a lot of known issues. We'll see.
