Add QQuickFontMetrics.

This will be a private C++ type that is exposed to QtQuick 2.4 as
FontMetrics.

[ChangeLog][QtQuick] Added QQuickFontMetrics, which provides a subset
of QFontMetricsF's API.

Change-Id: Iac31e5a555bd8f1dc0904b8de0408e5f1a402b25
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>