WIP: Package standard .desktop file in AppImage

[ferdnyc]

@TuxVinyards pointed out that the .desktop file in our AppImage doesn't contain the Categories= line required for it to properly show up in application menus. It was also missing a bunch of other fields, because instead of using the standard file from xdg/, the build-server.py script was generating its own abbreviated version.

But reading over the code for openshot-qt.wrapper in the AppImage, it seems like we could just use the standard file — it rewrites the Exec= line anyway, when copying it into the user's applications directory.

So, this PR is an experiment with just copying xdg/org.openshot.OpenShot.desktop into the AppImage, instead. (It's a branch in the project repo, so an AppImage will get generated by the builders.)

(Hopefully) fixes #3222

[ferdnyc]

Whoops, dropped a paren. Let's try that again...

[ferdnyc]

This PR also removes $(APPIMAGE_MOUNT)/usr/lib/mime/packages/openshot-qt, because:

1. It contains a hardcoded /usr/bin/ path
2. Nothing uses it, that I can find — on my (Fedora) system, it is the ONLY file in /usr/lib/mime/packages/
3. Everything works just fine without it, as far as I can tell.

I'd really like to get rid of it entirely (including removing it from the setup.py-generated installs), but I'm using the AppImage as a proving ground since I'm almost certain that it's broken there even if it DOES work elsewhere.

[ferdnyc]

First attempt failed miserably, not really sure why TBH. Trying again, but now rewriting the Exec= line on the way in.

Edit: That worked. Guess the Exec= line does matter!

[ferdnyc]

Going to merge this, so our AppImage .desktop file is generated with Categories and other goodness.