Normalize layers/groups management in TOC

[tdipisa]

Description

To maintain retro-compatibility with old maps, as reported here in #10246, with the new TOC there are still a few inconsistent but not blocking behaviors due to how the 'default group' is managed.

Loading a Map in one of the following scenarios:

• Importing a Map
• Opening an existing Map

and logic should intervene to automatically update the TOC internal structure to definitely fix retro-compatibility issues.

What kind of improvement you want to add? (check one with "x", remove the others)

• Minor changes to existing features
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Other useful information