config/input: validate xkb keymap before storing

This allows for an optional validation stage when storing an input
config. Currently, only the xkb keymap is validated. If storing the
delta input config will result in any invalid xkb keymaps, the input
config will not be stored and error will be populated with the first
line of the xkbcommon log.

sway/commands/input/events.c

@@ -101,7 +101,7 @@ static void toggle_wildcard_send_events(int argc, char **argv) {
 		} else {
 			toggle_supported_send_events_for_device(ic, input_device);
 		}
-		store_input_config(ic);
+		store_input_config(ic, NULL);
 	}
 }