Implement urgency base functionality

Introduces a command to manually set urgency, as well as rendering of
urgent views, sending the IPC event, removing urgency after focused for
one second, and matching urgent views via criteria.

sway/commands.c

@@ -153,6 +153,7 @@ static struct cmd_handler command_handlers[] = {
 	{ "swap", cmd_swap },
 	{ "title_format", cmd_title_format },
 	{ "unmark", cmd_unmark },
+	{ "urgent", cmd_urgent },
 };
 
 static int handler_compare(const void *_a, const void *_b) {