refactor both i3-nagbar starts into src/util.c

With this change, libev >= 4 is a hard dependency. It should be present
in all major linux distributions (even the latest ubuntu LTS).

include/key_press.h

@@ -10,6 +10,8 @@
 #ifndef I3_KEY_PRESS_H
 #define I3_KEY_PRESS_H
 
+extern pid_t command_error_nagbar_pid;
+
 /**
  * There was a key press. We compare this key code with our bindings table and pass
  * the bound action to parse_command().