fdo#65826 and fdo#73605: set a minimum size to start center

This minimum size calculated like this:
width: sidebar optimal width + width of text appearing in
the thumbnail view when no recent document is available.
height: menu width + optimal width of GtkBox containing
buttons.

Change-Id: Ic1fd6e33a1ec93e61318af02c09ee88909a76cf4
Reviewed-on: https://gerrit.libreoffice.org/7505
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>