Add support for resizing fonts to fit Text dimensions.

This adds a mode where if the content of a Text item doesn't fit
within its bounds the font size is reduced during layout until it
does or a minimum font size is reached.

Task-number: QTBUG-22832
Change-Id: I6198ef03899e2f21b32e313548966ef4b0e3bff1
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>
Reviewed-by: Yann Bodson <yann.bodson@nokia.com>