xhr util onload/abort

[dbauszus-glx]

The xhr util should assign the xhr object to the params object.

The abort method should then be called if the util is called with the same decorated object.

It should be possible to define a custom onload method in the xhr utils params argument.

The mapp library should not require to call directly on XMLHttpRequest web api but only through the xhr utils module.

The gazetteer and mvt xhr calls can be replaced with these utility enhancements.