GtkPrintOperationWin32: Separate custom property sheet

Instead of embedding the custom property sheet into the print dialog,
move it to a separate utility window, and leave in its place a button
that shows this window.
Auto-hide the window when the print dialog receives focus.
Position this window on top of the print dialog (specifically, on top
of the button that calls it).

This solves the problem where print dialog tab is not big enough for
the property sheet (we *can* resize the dialog, but it is unclear
how to get the size of the property sheet contents in advance; clearly,
gtk_widget_get_preferred_size() was not enough fort that). This can
also be forward-ported to GTK4, which does not support widget embedding
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