[IoT-Modelling-Tool] Undo/redo for connections

[schneijan]

At the moment, creating and deleting of connections between devices and components is not correctly tracked for the undo manager. This is the case because undoing an action or importing a model (which affects an already established connection) is again tracked as a new user action and thus pushed on the history stack. We may need something like a flag here in order to differentiate between actions executed by the user and actions performed by the import function.