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 anymore.