UI: Add way to exec std::function via invokeMethod

Allows the ability to use invokeMethod on the OBSApp object to execute
an std::function.  This allows deferring an std::function call to the Qt
event queue.

UI/obs-app.hpp

@@ -26,6 +26,7 @@
 #include <util/util.hpp>
 #include <util/platform.h>
 #include <obs-frontend-api.h>
+#include <functional>
 #include <string>
 #include <memory>
 #include <vector>
@@ -58,6 +59,8 @@ public:
 			const char *disambiguation, int n) const override;
 };
 
+typedef std::function<void ()> VoidFunc;
+
 class OBSApp : public QApplication {
 	Q_OBJECT
 
@@ -166,6 +169,9 @@ public:
 		translatorHooks.pop_front();
 	}
 
+public slots:
+	void Exec(VoidFunc func);
+
 signals:
 	void StyleChanged();
 };