Add as_node_to_file()
trivial: Export some more useful API from AsNode
Add as_node_remove_attribute()
Allow 'appstream-util convert' to work with two AppData files

This will provide a convenient way to upgrade AppData files from the old
standard to the new AppStream specification.
Support parsing .desktop.in files
Add as_app_to_file()
Add 'appstream appdata-from-desktop' to autogenerate a boilerplate AppData file