support app metadata in XML format

support app metadata in XML format

While the current text metadata format is good for human readability and
editability, it is difficult to produce and parse using code.  XML is a
widespread standard format for easy automatic parsing and creating, while
having decent human readability.

The .pickle for testing is a lightly edited version of the real metadata
for net.osmand.plus:

* comments were removed
* "NonFreeNet" was added as an AntiFeature