now we can deal with thread safety in presence of several transactions
being treated.

and more context and a clearer structure (thanks Axel for the feedback!)

Redefinition of a custom column is currently allowed due to lack
of checking and sort of works (last definition "wins"), but will
be checked for and will count as an error in the future.

A TTaskGroup represents concurrent execution of a group of tasks.
Tasks may be dynamically added to the group as it is executing.

Closes all connections to clients. As a rection, all opened window
should be closed. Currently worsk with CEF, Qt5 and local/remote
browsers

Invoked when current canvas version delivered to all clients.

Callback function called when file is created (or command fails)

One can call Canvas::SaveAs() several times, all specified files will be
created one by one asynchronously

It let configure more complex program arguments for starting browser for
the canvas, provide brief docu

By defauly, ROOT will try to crate local window with CEF or Qt5.
If not available, default browser will be started.
If place is specified, it should be program name like:
   canvas->Show("/usr/bin/opera");
   canvas->Show("firefox");

One could show canvas in several different places.

Contrary to SVG, it is binary format. 
It is transfered in base64 coding and then decoded.

When Canvas::SaveAs() method is called, following steps are preformed:
- canvas painter is created (if not exists)
- canvas painter waits JS client, which performs drawing
- sends command via websocket to create new SVG file
- receives SVG code via websocket and store it into the file

Thanks to Alberto Ferro for these developments.

thanks to Alberto Ferro!