[nem-en] Multiple project solutions

[vc]

> btw, can you explain why exactly do you need it? Is it
> - the "locking" issue, you mentioned earlier, but did not explain deeper
> - some memory / perf. savings
> - having InternalTypes from different projects the same, so you can
> test for equality between them

We mast share or copy types between projects (it's NamespaceTree) If
solution (in VS terms) contains two or many projects which dependency
between them. Before compilation assembly does not exist. And project can
contains some errors (which prevent make assembly from it). But dependent
project must have type information about it dependency project.

For each project we create Engine instance (and it private NamespaceTree).
We need opportunity to copy types from one project into another. Also we
need reload it types if dependency project has been changed.

In other words, we must share or copy type-infos from one project to another
and remove it if dependency project has been changed.