GUI: preparing to rm descriptors

Mikhail Svechnikovrequested to merge
cleanup_before_descr_changes into main

Currently in some SessionItem-based classes so called "Descriptors" are used. We decided that they are redundant and complicated to understand and use, so they should be eliminated from the parts of code under refactoring.

Renaming and reorganization of some functions is required before descriptors are removed. So these changes, not touching the logic, are here.

Edited by Mikhail Svechnikov

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