Save XML files with deterministic attribute order

Disable randomized QHash seed salt, so that QDomDocument will save XML
attributes in a deterministic (albeit arbitrary) order.

Now, saving an unchanged project yields an identical .kdenlive file
which can be checksummed, diff'ed, etc.

Reference: http://doc.qt.io/qt-5/qhash.html#algorithmic-complexity-attacks
Reference: http://stacksolve.com/3W1nJraYm18Q/qt-5-produce-random-attribute-order-in-xml.html