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.

common/ipc-client.c

@@ -111,7 +111,7 @@ error_2:
 	free(response);
 	free(payload);
 error_1:
-	wlr_log(WLR_ERROR, "Unable to allocate memory for IPC response");
+	sway_log(SWAY_ERROR, "Unable to allocate memory for IPC response");
 	return NULL;
 }