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
drawingML import: parse shape text run inside w:sdt and s:sdtContent

Change-Id: I6dc5939ae66967785cdc5dab318024b8cb17d1cd