GUI: JobModel is refactored

Mikhail Svechnikovrequested to merge
from_session_model into main

JobModel dependence from SessionModel was significantly reduced.

Job items were disconnected from JobModel's "root_item", they are stored in the vector now, not tree.

Read/write system of JobModel and JobItem was simplified and made more explicit.

Some minor things are also done for RealModel, but RealModel will be refactored next in a similar way.

Edited by Mikhail Svechnikov

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