Update GUI code to work with KSP 1.1 and Unity 5

[jwvanderbeck]

This changes the GUI handling to work properly under KSP 1.1 and Unity 5. So far it seems solid.

One additional change is required inside of MonoBehaviourWindowPlus but that doesn't seem to be a part of this project. In that class it is important that the OnGUIEvery() method call the base.

 internal override void OnGUIEvery()
 {
        base.OnGUIEvery();
        ddlManager.CloseOnOutsideClicks();
 }