Load/Save Common Files from/to source

[richardpawson]

(This will replace the current LoadContentsFromFile. Both actions will be available only on common files i.e. those with a UniqueRef).

Important: both methods must use a 'deferred function' on the IContext as loading/saving would be non-pure functions.

The path to the file may be obtained from methods in the CommonFileDefinitions, plus the Name of the File, which will (shortly) include the file extension for common files.

[richardpawson]

Not now importabnt