Add first-time configuration guide for global hotkey on wayland

Wayland prevents global hotkey registration as part of its anti-keylogger crusade, which means that Guake's global toggle needs to be done by adding `guake-toggle` to the window manager's global keyboard shortfcuts. Currently this is not mentioned in the program, so I'd like to propose the following flow for the program:

1. Detect wayland by catching global hotkey binding failure
2. Pop a dialog to explain the situation and provide instructions on how to set the hotkey in keyboard settings.
3. Offer to unbind the global hotkey so that further messages do not appear.