Windows have an initial off-screen rect, so when a dialog is created and
shown before its parent window, in which it is centered, it would be
invisible to the user.
Fixes#3172.
Now that add() returns a WidgetType&, we can't rely on the parent of a
GUI::Dialog to still keep it alive after exec() returns. This happens
because exec() will call remove_from_parent() on itself before
returning.
And so we go back to the old idiom for creating a GUI::Dialog centered
above a specific window. Just call GUI::Dialog::construct(), passing
the "parent" window as the last parameter.
