feat: Added ability to control overlays via the RouterComponent

[st-pasha]

Description

1. Class _ActiveOverlays renamed into OverlayManager and move into its own file. The class is still marked as internal.
2. In GameWidget.controlled constructor the properties initialActiveOverlays and mouseCursor were missing, now added.
3. Created class OverlayRoute for encapsulating an overlay-handling logic within the RouterComponent.
4. New overlay builders can now be added to the game at runtime.
5. The OverlaysManager._activeOverlays property is now a list instead of a set, so that the overlays would be displayed in the specific order requested by the user.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have read the Contributor Guide and followed the process outlined for submitting PRs.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples.

Breaking Change

• [-] Yes, this is a breaking change.
• No, this is not a breaking change.

Related Issues

Closes #1762

[spydon]

Very nice!