Rename/Clarify `Application.Toplevels`/`Top` Terminology

[tig]

Rename Applicaiton.Top to something that better represents "the part of the application that starts with a top-level View". Consider names like Views, ViewHierarchy, or similar.

Background:
The current Toplevel class and Top property naming is confusing. The goal is to eventually get rid of the current Toplevel implementation (see Issue #2491) and have clearer terminology.

Considerations:

• What should Application.Top be renamed to?
• How does this relate to the broader Toplevel refactoring effort?
• What terminology best describes the view hierarchy concept?

Tasks:

• Propose new naming convention
• Create migration plan for public API
• Consider deprecation strategy for old names
• Update all internal references
• Update documentation
• Coordinate with Toplevel refactoring effort (Issue Master Issue: Get rid of Toplevel - Introduce Runnable and Overlapped instead #2491)

Acceptance Criteria:

• Clear, intuitive naming for view hierarchy concepts
• Backward compatibility maintained (deprecation warnings)
• Documentation updated
• All tests pass

Related to:

• Issue Reorganize Terminal.Gui/App Folder Structure #4329
• Issue Master Issue: Get rid of Toplevel - Introduce Runnable and Overlapped instead #2491 (Toplevel refactoring)