Add libnotify support for linux client
Also unify notification management
Make libnotify mandatory for the client. It is installed on 100% of Arch
Linux systems and on 70% of Debian systems. I'd guess the remaining
Debian systems are servers.
Detect dependent libraries of `libnotify` using `pkg-config`. Remove
library-specific code from the game module. Decrement refcount for
libnotify notification object before leaving the function.