Raise window "properly"

[d2718]

Currently to "raise" one of the three windows in response to user input (Alt-A, Alt-Z, or Return), it is hidden (WidgetExt::hide()) and then immediately shown (WidgetExt::show()) again.

1. This seems hacky. It seems as though there should be a way to just raise a window. The official FLTK documentation suggests that Fl_Window::show() is the way to do this. However, WidgetExt::show() doesn't seems to work, and neither does DoubleWindow::platform_show().
2. This works okay on my Debian system running i3; the flicker of the windows disappearing and reappearing is barely noticeable. However, Win10 rapidly fades the involved window out and back in (and seems to shrink/grow it a little, or something, too), making this pretty obnoxious.