[Lazarus] JSON Dataset in FCL-DB.

[Michael Van Canneyt]

Hello,

I just committed a JSON dataset in fcl-db.

This is a dataset which stores its data in JSON objects (as in fpjson) in memory. 
The records are in a JSON array, each record can be a JSON object or array.
(not mixed)

The dataset is read-write. It can be used to create JSON data from scratch: 
just add definitions to the fielddefs properties, and call 'Open'. At the end, 
call SaveToStream().

There are 2 descendents which read/write ExtJS JSON data packages (Objects 
or Arrays) from streams but they are usable to handle any kind of JSON data. 
Currently, the dataset does not yet support propagating changes (deltas) back to a server.

I would be glad to recieve feedback, comments.

Michael.