Reorganize Terminal.Gui/App Folder Structure

[tig]

Update the folder structure in ./Terminal.Gui/App to match the logical organization of Application capabilities.

Current Structure:

• Files are organized by functionality (Driver, Keyboard, Mouse, etc.)
• All Application partial classes are in the root App directory

Proposed Structure:
Consider organizing into subdirectories:

./Terminal.Gui/App/
  ├── Lifecycle/
  │   └── Application.Lifecycle.cs
  ├── Driver/
  │   └── Application.Driver.cs
  ├── Input/
  │   ├── Application.Keyboard.cs
  │   └── Application.Mouse.cs
  ├── Views/
  │   └── Application.Toplevel.cs
  ├── Run/
  │   └── Application.Run.cs
  ├── Screen/
  │   └── Application.Screen.cs
  ├── Navigation/
  │   ├── Application.Navigation.cs
  │   └── ApplicationNavigation.cs
  └── Popover/
      ├── Application.Popover.cs
      └── ApplicationPopover.cs

Acceptance Criteria:

• Folder structure reflects logical grouping of capabilities
• Build and tests continue to pass
• Project files updated accordingly
• Documentation updated to reflect new structure

Related to: Issue #4329