Martin Dobias 137eb3a0f9 Make layer tree implementation independent from QgsProject::instance()
Another bit in the project refactoring work to get rid of dependencies
on QgsProject singleton.

Reading of layer trees from XML is now split into two phases:
1. read XML and keep layer IDs
2. resolve layer IDs to QgsMapLayer instances (using QgsProject)

There are convenience methods to do both phases in one go.
2017-01-27 09:29:04 +08:00
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