Only in such case updates will be delivered to clients - also in batch
mode

To create different images canvas will always start headless browser -
at least now. To generate many files same job will be used.

Batch connection can be used for some dedicated jobs and will be cleaned
up after some predefined time. Normal connections are dedicated for
graphics. It should be allowed to have both of them.

Instead of "start and pray" now WebWindow::Show() returns identifier,
which can be checked for timeout. If application was not started
correctly, one can retry or just simply return with error flag. 

Will be used in all kind of synchronous actions - like image production.

Now URL string contains key which is generated for the new starting
process. It should unique identify connection id. This id can be used in
further operations.

Allows to refuse connection on very early stage

Introduce correspondent constructor signature for contained classes

created

if sender thread was started, one have to call join it - even when
thread function was normally returned

If does not respond after specified time - kill it

Probably, one could create special class for canvas

This items not need to be shared outside connection mutex

Does not work for now, may be later better solution will be found

In normall condition headless chrome immediately exits after page and
all scripts are loaded. To keep it running, special flag was used
enabling debugging mode and external ports. 

Much better solution - append special script to the page which never
loads. Chrome will keep running forever - until we kill it.

Required for headless browser.
Must be carefully done, while requests processing performed from
THttpServer thread

This is special requests, which does not processed immediately but hold
for the long time by the receiver. 

For the moment used in RWebWindow to hold headless browser until
processing is done

Some browsers (like newer Chrome) make fragmentation on chunks of 120KB. 
In the handler one need to combine chunks together before providing to
user code

Then connection is closed by client or when batch job is completed,
one can try to delete temporary profile for that job, which can be
about 30 MB.

Otherwise firefox always tries to access default profile, 
which is in many cases not a proper way. 

It replaced --remote-debugging-socket-fd from previous chrome versions.
It is workaround to let chrome runs longer in batch mode
Probably better solution can be implemented - with support from the
server side

Added --version to TApplicattion

Included commit hash and architecture

removed mistake

...as the presence of a datasource changes the type of RInterface.

It triggers wrong behaviour of Jenkins parser together with CDash adding ANSI escape symbols: NON-XML-CHAR-0x1B][0m[NON-XML-CHAR-0x1B]

GetSize is deprecated from 3.3
(https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_GetSize) and
the new recommendation is to use PyUnicode_GetLength.

Ensure that variables are actually read from the TTree...

Based on the solution exposed here: https://github.com/root-project/root/commit/9339de9e644b06d5f77607f83aea118969cab7b6