Minor refactor of input manager

The input manager is a singleton object. Passing the sway_input_manager
argument to each of its functions is unnecessary, while removing the
argument makes it obvious to the caller that it's a singleton. This
patch removes the argument and makes the input manager use server.input
instead.

On a similar note:

* sway_input_manager.server is removed in favour of using the server
global.
* seat.input is removed because it can get it from server.input.

Due to a circular dependency, creating seat0 is now done directly in
server_init rather than in input_manager_create. This is because
creating seats must be done after server.input is set.

Lastly, it now stores the default seat name using a constant and removes
a second reference to seat0 (in input_manager_get_default_seat).

sway/server.c

@@ -153,7 +153,9 @@ bool server_init(struct sway_server *server) {
 	server->dirty_nodes = create_list();
 	server->transactions = create_list();
 
-	input_manager = input_manager_create(server);
+	server->input = input_manager_create(server);
+	input_manager_get_default_seat(); // create seat0
+
 	return true;
 }