UI: Add signal for when theme has changed

UI/obs-app.cpp

@@ -994,6 +994,7 @@ bool OBSApp::SetTheme(std::string name, std::string path)
 	setStyleSheet(mpath);
 	ParseExtraThemeData(path.c_str());
 
+	emit StyleChanged();
 	return true;
 }
 

UI/obs-app.hpp

@@ -165,6 +165,9 @@ public:
 	{
 		translatorHooks.pop_front();
 	}
+
+signals:
+	void StyleChanged();
 };
 
 int GetConfigPath(char *path, size_t size, const char *name);