Rename weld::Dialog::widget_for_response to
weld::Dialog_button_for_response as it specifically
returns a weld::Button, not a generic weld::Widget (other than GTK's
`gtk_dialog_get_widget_for_response` [1] which
returns a GtkWidget).
[1] https://docs.gtk.org/gtk3/method.Dialog.get_widget_for_response.html
Change-Id: I3b1dc34c0af752853551d2ebb706109f2214720d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/177376
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
