Use strndup so we can use expat-type parsers without an additional copy
trivial: Add a self test for as_app_subsume()
Add as_app_get_description()
Add functionality to search values in the AsApp
Add a way to store the icon path in the AsApp
Add as_app_get_compulsory_for_desktops()
trivial: Remove dead code
Make as_app_add_language() more friendly for expat-style parsers

We use <lang percentage=100>en_GB</lang> so the language itself has to be the
text with the length, although used internally as the key.