RF: Don't create new actions for toggling control panels - use the existing actions that are automatically added to the `MRSView`

Paul McCarthyrequested to merge
paulmc/fsleyes-plugin-mrs:rf/actions into master

This change is needed to work around an upcoming breaking change in FSLeyes - in the next version, manually created Action objects must be registered via an addAction method.

The changes here should work with existing FSLeyes versions though.

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