adding a weld::Calendar and a pretty menubutton to access it, sidestepping the
difficulty of abusing a spinbutton to select a date. The prettiness is wasted
on this hard to find obscure dialog

Change-Id: I51d461fe0220f947c106d96965e6422b4b26575b
Reviewed-on: https://gerrit.libreoffice.org/67863
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>