GUI: FitParameterModel and fit items are refactored

Mikhail Svechnikovrequested to merge
fitModel_and_fitContainer into main

Now free of SessionItems: FitParameterModel, FitParameterContainerItem, FitParameterItem, FitParameterLinkItem.

FitParameterContainerItem, FitParameterItem, FitParameterLinkItem and their properties are now QObject-based and they hold their parent-child relationships.

Unfortunately the MR is quite big. This is due to the refactored parts of the code are tightly coupled and they didn't work in intermediate state.

Edited by Mikhail Svechnikov

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