Delete Chandler project.

The console test application, Chandler, has lived up its usefulness and
thus can be deleted as a clean up effort.
Add tests project skeleton.
Add Moq dependency to tests project.
Test model default property values.
Test promoting model to shared state.
Prevent unnecessary Errors property change notifications.

Currently validation code notified about Errors property being changed
every time it was run. This shouldn't have happened.
Move common test logic to Test baseclass.
Test model lookup.
Switch to NuGet NUnit.

Due to some very weird behaviour from Xamarin.Studios, it was possible
to use the bundled newer NUnit.Framework for some time. Probably after
reopening the solution it was unable to find the bundled version and
used system nunit, which was older.

Using NUnit from NuGet solves such Xamarin tools instability problems.
Refactor model lookup code into ModelManager.

ModelManager is responsible for caching and managing shared models.
Having it as a separate entity enables us to more easily unit-test the
models as we can easily wipe the caches and run the next test.

Some of the old static methods still remain as convenience methods.
Broadcast model change before validation.

Send the global ModelChangeMessage before the validation logic has
kicked in to avoid sending validation errors before the property change
message.
Repurpose ModelLookupTest as ModelManagerTest.
Test model validation.
Fix validation error extraction.

Unit tests revealed (from 6bc8727) that there were some problems with
updating existing errors values. Refactored ValidationContext a bit to
fix the problems.