Remove gtk_dialog_run() from GtkPrintUnixDialog

Remove gtk_dialog_run() from GtkPrintUnixDialog

Replace it with an explicit nested main loop, as we need to block the
signal handler currently being emitted depending on the response of the
overwrite confirmation dialog.

This is a bit of a hack, and the only reason we need it is that the
print dialog will load the last used path as the output file name, when
printing to a file; this means that, in theory, it would be possible to
press Print without selecting a file, and accidentally overwriting an
existing file.

It would be much simpler if we did not store the last used path, and
always explicitly asked the user to select a file; this would avoid
destructive actions, and would allow us to rely on the overwrite
confirmation dialog right inside the file chooser.