Fix exception when rotating skewed item.

[jongleur1983]

Work in progress to fix issue #36 - starting with a failing unit test.
The test reproduces the described issue (I hope) and fails as expected, yet by throwing the same exception described by @mnusr
Not sure I'll get a solution, but I'm working on it.

Update: The issue in the demo app is solved.
The unit test doesn't, yet - so I might be doing something wrong there.

But: The demo app has a strange rotation behaviour now - not sure yet how the rotation angle is calculated from mouse input, this might be broken somehow on Skewed items.
I don't think that's based on the changes in this PR, but wasn't just not visible before.

[jongleur1983]

Mentioning you, @jogibear9988 as you answered in the ticket before and "promised" to have a look at the fix:

I revoke from fixing the underlying issue in the UI as I fail to get around the overall UI and I don't think I'm going to invest much more time in this project in the next time.
Therefore I remove the WIP-marker from this PR and leave it as it is.

I'm happy to change what's necessary when I know what to do, so any hints are welcome.

AppVeyor fails for two reasons:

• my new unit test is failing. This happens as the test is more an integration test right now and I don't know how to write a proper test without changing much of the structure. The problem is that although the ModelTools get an IDesignPanel it casts to the concrete DesignPanel class internally - which fails with a mock in the tests. I'd consider this bad design but was afraid to just relax the code in ModelTools accordingly (ignoring the designPanel-stuff when it's not the concrete class instead of enforcing that).
• the other failing unit tests fails (at least for me locally) in master as well, so it seems to not be caused by my changes.