Add small CLI tools performing specific libappimage operations

[TheAssassin]

Sometimes it might be beneficial to offer interfaces additional to an API. A very simple interface type is CLI. It's used excessively by shells but can be used from almost any programming language; for API usage, one has to use C/C++.

We should consider providing some small wrapper tools that perform specific tasks for use by tools or scripts which can't afford linking to libappimage:

• appimage-register-in-system my.AppImage
• appimage-unregister-in-system my.AppImage
• appimage-extract-thumbnail my.AppImage > thumb.png
• etc.

The binaries might be shipped in separate libappimage-bin packages, like most library provided tools are.

[darealshinji]

Here's a small tool that allows to use most of the API from command line: https://gist.github.com/darealshinji/c126c37013b088e7d28e8d5ac7447fb8

Examples:

• libappimage-bin register-in-system my.AppImage
• libappimage-bin unregister-in-system my.AppImage
• libappimage-bin extract-file-following-symlinks .DirIcon "$PWD/thumb.png"