Add extractor for AppImage files
Only a few properties currently can be mapped more or less to the existing
values of the "Property" enum, ideally for the future can be extended.
"dump" test util and Dolphin both now show the extracted metadata for
Reviewers: Baloo, astippich
Reviewed By: astippich
Subscribers: TheAssassin, astippich, broulik, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D18450