Add first-time configuration guide for global hotkey on wayland

[Davidy22]

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.

[davidhaley]

I was trying to find out why F12 would not open/close Guake after logging into my desktop environment (Gnome), and your post lead me to an answer. Thank you!

For others, the solution was to add a custom shortcut in Gnome's keyboard settings (open the launcher, open settings, go to the Keyboard tab, and add a custom shortcut with guake-toggle as the command)