Include stream as tempate in XML Template

[Maxim Reznik]

We need a way to avoid expression substitution in nested stream of an template. It's error prone to have this as default. See #426 as example.

I propose to add an extra directive

<mtl:include expression="stream"/>

to explicitly force substitution in the stream from given variable.

[Maxim Reznik]

In 5817:

Add mtl:evaluate to XML Processor

Refs #461, #479

[Maxim Reznik]

Duplicate of #461

[vadim.godunko]

Milestone renamed