Refine LiveFile and LiveJson usage model

[3Nigma]

LiveFile (inherited by LiveJson) is exposing a getContents() method to retrieve the underlying content as a Promise<Uint8Array>.

We need to have:

• add a parameter to getContents called update which is a boolean defaulting to false that, when set to true does a FileContentsQuery network action, updating the underlying content in the process. If it's false, it retrieves the stored data in Uint8Array format.
• both LiveJson and LiveFile should use the query result to update it's internal data reference
• Implement LiveJson.toString() which will stringify its data
• Implement LiveFile.toString() which will return its internal data (if it's a string) or <binary File content> if the data is a Uint8Array.

toString() implementations will allow for fluent formatting in structures such as This is the file: ${liveFile}.