Improved logical and physical DPI APIs.

Improved logical and physical DPI APIs.

Made physicalSize() return QSizeF instead, to prevent rounding errors.
Added logicalSize() as the base to compute font pixel sizes instead, and
added convenience functions in QScreen to access the logical and
physical sizes and DPI metrics.

Task-number: QTBUG-21736
Task-number: QTBUG-21737
Change-Id: Ic705dc98eb3632617659e65a0c9a552673dc0c65
Reviewed-on: http://codereview.qt-project.org/5888
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>