Don't assume there is an active connection when loading devices list

Instead listen to Controller::activeConnectionChanged and reset model
when the active connection is changed.

Fix #413

src/devicesmodel.h

@@ -32,5 +32,6 @@ public:
     Q_INVOKABLE void setName(int index, const QString &name);
 
 private:
+    void fetchDevices();
     QVector<Quotient::Device> m_devices;
 };