Add extractor for AppImage files

Summary:
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.

Test Plan:
"dump" test util and Dolphin both now show the extracted metadata for
AppImage files.

Reviewers: Baloo, astippich

Reviewed By: astippich

Subscribers: TheAssassin, astippich, broulik, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18450