Add love.themechanged() event.

Called when the system theme is changed (e.g. switching from light theme to dark and vice versa).

src/modules/event/sdl/Event.cpp

@@ -573,6 +573,9 @@ Message *Event::convert(const SDL_Event &e)
 	case SDL_EVENT_LOCALE_CHANGED:
 		msg = new Message("localechanged");
 		break;
+	case SDL_EVENT_SYSTEM_THEME_CHANGED:
+		msg = new Message("themechanged");
+		break;
 	case SDL_EVENT_SENSOR_UPDATE:
 		sensorInstance = Module::getInstance<sensor::Sensor>(M_SENSOR);
 		if (sensorInstance)

src/modules/love/callbacks.lua

@@ -150,6 +150,9 @@ function love.createhandlers()
 		sensorupdated = function (sensorType, x, y, z)
 			if love.sensorupdated then return love.sensorupdated(sensorType, x, y, z) end
 		end,
+		themechanged = function ()
+			if love.themechanged then return love.themechanged() end
+		end,
 	}, {
 		__index = function(self, name)
 			error("Unknown event: " .. name)