Remove this bit of code from StyledEdit

....as we're gonna do the work in BFilePanel instead.
Create a BWindow::AlertPosition() method and use it

...to position alert's and open/save dialogs nicely inside of the parent window,
or if that is unavailable, the screen frame.

AlertPosition() is private (for now) but BAlert and BFilePanel are BWindow's friends so
BWindow allows those classes to touch it's privates.
Style fix in BAlert, some sort of strange spaces got in there
Fix #9726 Revert CenterIn() and CenterOnScreen()

... back to their previous void returning roles. AlertPosition() is used instead to
check that an alert fits within the sides of the screen and all that.

Also add another CenterOnScreen() method that takes a Screen ID
so you can center a window on another monitor that the one it is currently on
(theoretically someday anyway).