Replace child control count/index APIs with Controls() snapshots

Replace the direct-child inspection APIs on Window, ContainerControl, ScrollBox, and GroupBox with a Controls() As Array(Of Control) method.

This lets users enumerate controls and get the count from one API, for example Window1.Controls().Count, instead of needing separate CountControls() and ControlAt(index) methods.

The returned array is read-only from the owner control's perspective: callers should treat it as an inspection snapshot, and should use Add(control), Remove(control), and, where available, RemoveAll() to change child controls.

The controls inside the returned array are still the live control instances.