Add undocumented event functions - HasFrameBounds and OnGetFrameBounds

[citizenmatt]

Add the following undocumented event functions to Editor derived classes:

• bool HasFrameBounds()
• Bounds OnGetFrameBounds()
• void OnPreSceneGUI()

Confirm when they were added, and figure out some help text.

Might also be worth checking other usages of GetMethod in the reference source.

[citizenmatt]

See also #986. Includes some of:

• void Editor.OnSceneDrag()
• void EditorWindow.ModifierKeysChanged() - docs
• void EditorWindow.ShowButton(Rect) - docs
• void EditorWindow.OnAddedAsTab()
• void EditorWindow.OnTabDetached()
• void EditorWindow.OnBeforeRemoveAsTab()
• void EditorWindow.OnBecameInvisible()
• void EditorWindow.OnDidOpenScene()
• void EditorWindow.OnBecameVisible()