Easier placement of stages with positioning anchor

[Michael Strauß]

I've prepared a small enhancement for Stage that allows applications
to show a stage relative to a reference point, similar to how popups
work. This is a useful addition for a potential system tray support
(where windows are often placed relative to a system tray icon), but
also comes in handy for multi-window applications.

Comments are welcome.

PR: https://github.com/openjdk/jfx/pull/1986