Add FontDialog to QtQuick.Dialogs

As with FileDialog, it tries QPA, then QFontDialog, and falls back to
a QML implementation (which is also provided here) if neither type of
native dialog is available.

The update of plugins.qmltypes files will be in a separate commit.

Task-number: QTBUG-31852
Done-with: Shawn Rutledge <shawn.rutledge@digia.com>
Change-Id: I066ebbcf44c413af26020ddf8414252b99f5218b
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>