Change usages of GUI.enabled to EditorGUI.Begin/End-DisableGroup

[StanleyGoldman]

As @CapnRat commented here:

Should get into the habit of using EditorGUI.Begin/EndDisabledGroup rather than setting GUI.enabled directly. Those utility methods handle nesting properly. (There's also EditorGUI.DisabledScope, for an IDisposable way of doing things.)

[MunchyYDL]

I will look at fixing those.

[StanleyGoldman]

Cool beans. Let us know if you need help.
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