DB editor: load state should select the loaded entities

[jkiviluo]

All the entities that "load state" (graph view) gets, should be also selected in the entity tree after loading. Otherwise it's impossible to continue modifying this selection.

Discussed in https://github.com/spine-tools/Spine-Toolbox/issues/2449

[PiispaH]

I messed around with this a while back in this branch. Got it somewhat working, there were still some issues with selecting entities and entity classes at the same time. After initially trying this, the db editor has changed quite a bit, but at least some of the code from the branch should be salvageable. Doesn't seem to work anymore as is after I merged master into it.

Edit: The current issues with this seem to stem from master: https://github.com/spine-tools/Spine-Toolbox/issues/2903