Ensure SnapToGrid only appears when GridSnapping is enabled LYN-2302 (#1362)

* updates to better support showing/hiding viewport ui when grid snapping is enabled/disabled

* connect up editor settings callbacks

* minor polish changes

* api rename

* updates following review feedback

Code/Sandbox/Editor/EditorViewportWidget.h

@@ -24,6 +24,7 @@
 #include "Objects/DisplayContext.h"
 #include "Undo/Undo.h"
 #include "Util/PredefinedAspectRatios.h"
+#include "EditorViewportSettings.h"
 
 #include <AzCore/Component/EntityId.h>
 #include <AzCore/std/optional.h>
@@ -571,6 +572,9 @@ private:
 
     AzFramework::EntityVisibilityQuery m_entityVisibilityQuery;
 
+    SandboxEditor::GridSnappingChangedEvent::Handler m_gridSnappingHandler;
+    AZStd::unique_ptr<SandboxEditor::EditorViewportSettingsCallbacks> m_editorViewportSettingsCallbacks;
+
     QSet<int> m_keyDown;
 
     bool m_freezeViewportInput = false;