Add support for .metainfo.xml files

[Korne127]

Previously, appimagetool has expected a .appdata.xml file, although that format has been deprecated and .metainfo.xml should be used for new projects. This Pull Request fixes this issue. It adds support for .metainfo.xml files. With this PR, appimagetool will accept both .metainfo.xml and .appdata.xml files. This PR also adds a warning when using a .appdata.xml file that the file extension should be changed to .metainfo.xml.

Additionally, the link to the generator hasn't worked anymore. This PR replaces it with a working link to an AppStream generator.