Fixed Unity 2019 having no GUI among other issues.

[Henry00IS]

This commit fixes #234 and #233 and provides complete Unity 2019 support.

• The !UNITY_2018_OR_NEWER was replaced by !UNITY_2018_1_OR_NEWER that 2019 recognizes properly.
• SceneView.onSceneGUIDelegate has been replaced by SceneView.duringSceneGui.
• UnityEditor.StaticEditorFlags.LightmapStatic has been replaced by UnityEditor.StaticEditorFlags.ContributeGI.
• sceneView.m_SceneLighting has been replaced by sceneView.sceneLighting.
• SceneView.SceneViewState.Toggle has been replaced by SceneView.SceneViewState.SetAllEnabled.

EditorHelper.SceneViewHasDelegate has been deprecated. SceneView.duringSceneGui is an event and C# enforces that it can only be mentioned in conjunction with a += or -= operator. In order to determine whether we are already subscribed I decided to create a boolean with the [NonSerialized] flag to ensure that any script recompilations will reset it.

Essentially this:

/// <summary>
/// Used to determined whether we are subscribed to the "During scene GUI" event.
/// Recompilations will reset this variable to false and we can rebind.
/// </summary>
[NonSerialized]
private bool isSubscribedToDuringSceneGui = false;

if (!isSubscribedToDuringSceneGui)
{
    // Then resubscribe and repaint
    isSubscribedToDuringSceneGui = true;
    SceneView.duringSceneGui += OnSceneGUI;
    SceneView.RepaintAll();
}

I tested this new code on Unity 5.3, 2017, 2018 and 2019. Works perfectly.