obs-frontend-api: Add functions to get last saved files

Adds functions to the frontend api to get the last recording,
screenshot and replay buffer saved.

UI/window-basic-main-screenshot.cpp

@@ -54,6 +54,8 @@ ScreenshotObj::~ScreenshotObj()
 				QTStr("Basic.StatusBar.ScreenshotSavedTo")
 					.arg(QT_UTF8(path.c_str())));
 
+			main->lastScreenshot = path;
+
 			if (main->api)
 				main->api->on_event(
 					OBS_FRONTEND_EVENT_SCREENSHOT_TAKEN);