GH-1559 Fix FTB icons

This was caused by separation of GUI and logic. Now logic has an interface that GUI implements.
It should be expanded upon later.

application/MultiMC.cpp

@@ -344,6 +344,7 @@ void MultiMC::initIcons()
 	{
 		m_icons->directoryChanged(value.toString());
 	});
+	ENV.registerIconList(m_icons);
 }