Improvements to effectiveSizeHints
Improvements to effectiveSizeHints

- Make sure the implicit minimum size is (0,0) and not (-1,-1)
- Avoid potential crash in effectiveSizeHints.
- Small readability improvement

Change-Id: I0b08020099fd9e32f5d2bec323475c2b134439ec
Reviewed-by: Jens Bache-Wiig <9cea9d70b08f8210134ec5959114f72bad51ed1d@digia.com>

mizukusak/qtquickcontrols,richardmg/qtcomponents,drgvond/qtquickcontrols,jfaust/qtquickcontrols,mizukusak/qtquickcontrols,richardmg/qtcomponents,jfaust/qtquickcontrols,drgvond/qtquickcontrols,jfaust/qtquickcontrols,richardmg/qtcomponents,drgvond/qtquickcontrols,mizukusak/qtquickcontrols,kobolabs/qtquickcontrols,kobolabs/qtquickcontrols,kobolabs/qtquickcontrols