Alternative to global hotkeys for Wayland users

[paoloose]

Urn should support an alternative to global hotkeys for Wayland, like having command line flags to communicate via IPC with already running Urn timer instances.

Derived from issue #9 and comment #9 (comment)

The way to do global hotkeys in Wayland is … not to do global hotkeys. Instead, have command line options for the stuff you would usually hotkey; e.g. urn --start for starting the timer of an already running application instance. That can then be assigned as a “global hotkey” through the window manager / desktop environment.