Speed up Text construction with RichText

Setting the page size to empty can short cut some unnecessary layouts
triggered by QTextDocument/QTextDocumentLayout prior to the Text item
requiring a layout.

This rougly halves the construction time a Text item with RichText.

Task-number: QTBUG-19728
Change-Id: I03e6d293d8e5ac40046aef41171ca5009b718546
Reviewed-by: Martin Jones <martin.jones@nokia.com>