[Pas2js] Data Abstract support

[Michael Van Canneyt]

Hi,

As a small Christmas gift, I committed support for Remobjects Data Abstract
in pas2js: The TDAConnection and TDADataset classes. There is a demo as well.

I put the demo online on

https://www.freepascal.org/~michael/dataabstract/sampleda.html

It is a simple rewrite of the Remobjects Sample application for Remobjects
for Javascript.

You can see the sources and how simple it is to create data applications
using well-known techniques from FPC/Delphi.

The sources can also be used as good examples of how to declare and use
external classes.

Currently read-only but read-write is planned "real soon".

Enjoy,

Michael.