QmlJS: Split Context and ScopeChain.

Context is created by Link and has information about imports
for all Documents in a Snapshot.

ScopeChain represents how lookup is done at a specific place in
a Document.

Change-Id: I874102d57bbaf1a497fa3f27633bed6ee75dcf10
Reviewed-on: http://codereview.qt.nokia.com/1694


Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>