Include the promised `extract` and `inspect` functions into `appimagetool`

The *`appimagetool --help`* utility prints:

appimagetool [OPTION...] SOURCE [DESTINATION] - Generate, extract, and inspect AppImages

However, the *`inspect`* and the *`extract`* features are currently only promises about a bright future, but not implemented. (Unless you count the *`--list`* in as part of *inspecting* an AppImage...)

I've heard arguments to drop even the *plans* to include functionality for extracting (and possibly inspecting) AppImages. The argument is that type 2 AppImages already support the *`--appimage-extract`* option, they can "self-extract".

Ok, then rename this one to *`appimage-builder`* or some such...

However, I strongly request to have an *independent* tool to extract and inspect AppImages. These are my reasons:

1. I want a tool which I **trust more** to investigate an AppImage I just downloaded and have not confidence to make it executable to run it. I strongly feel we should have the ability to make it very easy to *extract AppImages which are not set to self-execute* yet!
1. It could potentially help to extract AppImages which are buggily implemented and cannot self-extract.
1. It should be able to extract type 1 AppImages as well (who knows, how long these will still be around??)
1. You promised the function in above quoted *`--help`* output.
1. You are working on a library which could be used by third-party applications (like Distro App Stores or Software Centers) -- so why not use that same library in all appriate AppImageKit tools?
1. If you *"Eat Your Own Dog Food!"* -- you'd notice many bugs which sneak into a release faster as well...

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The *inspect* part of this should make it easy to check for some common problems and shortfalls which AppImage packagers still fall prey of. It should also enable users to collect relevant data points to report to an AppImage packager when they have a problem.