VCL positioning

Window coordinates are parent relative, with exception of maState fields, they
have screen coordinates.

Every window has a parent window, usually the client area of an existing window,
otherwise the desktop window.

When a window is to be sized/moved, SetPosSize is called.
Since coordinates are the position/size of client area, we need to calculate
current window frame size/position and map to screen coordinates using the
owner window position as offset.