Allow nested IContexts and use it to give extra editor windows a context

The extra editor windows need to have editor manager context, otherwise
shortcuts (like ctrl+tab) do not work in them if e.g. projects mode is
active. Doing this via add/removeAdditionalContexts would be non-trivial
and error prone, so adding a context to the extra window is more
convenient. Since editors themselves already define a context, we need
to allow nesting of contexts.

Change-Id: I244eca53ebd665fd4d8fe7531e8ff701ed0b40b2
Reviewed-by: David Schulz <david.schulz@digia.com>