Enumerate application windows

[dbwiddis]

We should add code to enumerate/iterate application/console windows on the "desktop" (or equivalent) of GUI-based operating systems.

On Windows, the EnumWindows() function is probably the best starting point, combined with EnumChildWindows. It's also possible to only use the child windows variant, starting with null.

On macOS, the Core Graphics API function CGWindowListCopyWindowInfo() is the best. I have written code to do this in this StackOverflow answer.

On *nix, there are probably X11 functions which are appropriate to use, but this will need research.

Important information to include in the "Window Handle" Object:

• Window Title (if any)
• Title of owning process (default for window title)
• PID of owning process
• Layer/Relationship (which window is on top: Z-Order on Windows)
• Whether the window is visible
• Window position and dimensions/bounds
• Window transparency/alpha

[dbwiddis]

JNA has a WindowUtils class with much of this already done for Windows, macOS, and X11, so this is probably easily accomplished by generalizing those method results into Oshi objects (and possibly testing the X11 code in the Unix environment).