CATROID-1056 Fix wrong variable references after undo

[mstoeg]

https://jira.catrob.at/browse/CATROID-1056

When loading the old values of the variables after an undo a deep copy of all the uservariables was restored. This leads to the problem that there are now two instances of the same uservariable (one in the uservariable list of the project and one that is actually stored within the bricks). In order to fix this after performing an undo the uservariables are not copied in a deep manner but just the values are restored (so that the references do not change).

Additionally changed the test FormulaEditorUndoTest to also check if the uservariable references stay the same after an undo.

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[dandrawes]

Done in PP!

[wslany]

Thanks!!