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>
Related fdo#65826: Set StartCenter's min width to display one column of docs

If there is document to display. Otherwise set minimum to the width
of greating background text.

Updating minimum size is needed only when the list is cleared.

Change-Id: Ic089571fd21d5ab9ded0f4cde7e1aa72fd508d32