Add option to extend scene sequences (#121)

Extending a scene sequence allows for more complex switching setups, but should also simplify some setups.
Sequences like "A -> B -> A -> C", which previously were only possible by creating a copy of A, can now be specified in a single entry.
To extend a sequence either select the sequence you want to modify and click the extend sequence button or simply double click the entry.

src/headers/switch-generic.hpp

@@ -73,7 +73,7 @@ public:
 
 	static void swapSwitchData(SwitchWidget *s1, SwitchWidget *s2);
 
-private slots:
+protected slots:
 	void SceneChanged(const QString &text);
 	void TransitionChanged(const QString &text);
 	void SceneGroupAdd(const QString &name);