And move the QQuickDesktopIconProvider class to a separate header.

Change-Id: I16fb1d5f286ccf8ba6c671a12b843565c2b52a22
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>