Shells can be separated into two categories: top-level shells and secondary/dialog shells.
Shells that do not have a parent are top-level shells.
All dialog shells have a parent.
Use the following constructors to create top-level shells:
Shell()
Shell(int style)
Shell(Display display)
Shell(Display display, int style)
Dialog shells can be created using the following constructors:
Shell(Shell parent)
Shell(Shell parent, int style)