Add contentWidth and contentHeight properties to Text elements.

For Text and TextEdit this is a rename of paintedWidth and paintedHeight
both of which remain as synonyms of the content properties for
compatability.  For TextInput this is a new property.

Task-number: QTBUG-23691
Task-number: QTBUG-15160
Change-Id: Idbdc72fad34922be21b649ca45fc39b5e533ed1a
Reviewed-by: Yann Bodson <yann.bodson@nokia.com>