Replace wlr_log with sway_log

This commit mostly duplicates the wlr_log functions, although
with a sway_* prefix. (This is very similar to PR #2009.)
However, the logging function no longer needs to be replaceable,
so sway_log_init's second argument is used to set the exit
callback for sway_abort.

wlr_log_init is still invoked in sway/main.c

This commit makes it easier to remove the wlroots dependency for
the helper programs swaymsg, swaybg, swaybar, and swaynag.

sway/commands/output.c

@@ -28,7 +28,7 @@ struct cmd_results *cmd_output(int argc, char **argv) {
 
 	struct output_config *output = new_output_config(argv[0]);
 	if (!output) {
-		wlr_log(WLR_ERROR, "Failed to allocate output config");
+		sway_log(SWAY_ERROR, "Failed to allocate output config");
 		return NULL;
 	}
 	argc--; argv++;