Draft: Appimage plugin

Implements basic support for AppImage format (mentioned in issue: #1045 (closed))

It is based on an attempt of this implementation written by @probonopd (available here: https://gitlab.com/probono/gs-plugin-appimage)

Features:

• Opening AppImage files with gnome-software
  • Parsing data from embedded .desktop file
  • Parsing data from embedded appdata file
• Installing (registering) AppImage files
  • Moving to ~/Applications directory during installation (maybe make it optional?)
• Uninstalling (unregistering) AppImage files
  • Remove uninstalled file
• Listing installed AppImage files
• Updating AppImage files
• Installing AppImage files from web (using AppImageHub https://github.com/AppImage/appimage.github.io)

Other TODO-s:

• Fix bugs from base code
• Remove unnecessary logs and notes from code
• Fix pipeline
• Add Copyright notice
• ? Format code according guidelines