API for querying the window hierarchy of a display

[hgsfghs]

Haiku currently lacks a public interface for applications to query the window tree of the display, making simple tasks such as determining the frame rectangle of the window underneath the pointer difficult.

An ideal interface would be similar to the private `get_window_order' interface in WindowInfo.h. It should return a list of BWindows that are visible on the current workspace; that list can contain BMessengers (instead of BWindows) for windows that were created by other applications.