Fixes #509

The shift key gets ignored by the global Keybinder. That's not a bug in the library, it's a misconfiguration.
By default, keybinder uses "cooked accelerators".

I quote from the issue comments.

The best documentation I found is the comments in the code. They are pretty good.
From this:

"Cooked" accelerators use symbols produced by using modifiers such as shift or altgr, for example if "!" is produced by "Shift+1".

There is this C function

keybinder_set_use_cooked_accelerators

So yeah, we just had to disable them using this:

Keybinder.set_use_cooked_accelerators(False)

There is another issue about hide_window not working (#427). I doubt it's related to this, I think that one is about Wayland.