For "includes" we should use snapshot from "SnapshotUpdater"(project
specific snapshot, not global snapshot) instead of "global" snapshot.
The snapshot of the editor's SnapshotUpdater contains the documents
parsed with the appropriate include paths/defines etc. and should
therefore be used for the include hierarchy.

Change-Id: I6bbaf5040536b771d32697aad1db364758ff8382
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

Very simple tests for include hierarchy model:
* test if checked file includes one file
* test if checked file is included by one file
* test if checked file includes one file and is included by one file

Change-Id: I3688f9ddb86a569f780fc156bdfcf6b81a9fb0c1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>