Add optional display index parameter to Window.Show()

Opening a window on a specific display currently requires the documented multi-step pattern: choose a display with Display.At(index) or Display.Primary, then set the window's Left and Top from the display's available area before calling Show().

Add an optional zero-based display index parameter to Window.Show() so a window can be opened on a specific display in one call:

Var w As New Window
w.Title = "Tool Window"
w.Show(1)  # Opens the window on display 1 when available

Show() with no argument keeps its current behaviour. An index outside 0...Display.LastIndex raises a runtime error, matching Display.At().