Stop using QDomDocument::toString to avoid QHash random reordering

[glpuga]

This PR fixes a random reordering of keys in the xml output file that happens each time the file is saved from a new Groot process, even if no changes were made to the tree.

The issue can be replicated by creating a tree description xml file that has nodes with multiple input or output ports (the test_data/test_xml_key_reordering_issue.xml included in this PR, for instance). Open Groot, load the file and save a copy of it. Close and start Groot again, open the file again a save a second copy. Then compare the original with both copies using diff; there should be differences in the ordering of xml attributes.

This becomes an issue when the xml file is stored in a version control system such as git.

The random reordering happens because newer versions of the QDomDocument uses QHash, which in newer versions of the Qt library uses randomness to prevent certain types of DoS attacks.

There are provisions to prevent the random reordering from within the Qt library, but they are not the same in all versions of Qt.

This PR is based on an alternative solution posted here, which generates the xml contents recursively generating elements for each node.

This patch also adds a test for this fix.

[facontidavide]

when I saw that you included the unit test... I wanted to cry of happiness!

Thanks for this nice PR! i am testing it on my machine and merge it ASAP.