Update MIME Database During AppImage Build

[TheBrokenRail]

This helps prevent nasty errors like:

(zenity:859894): Gtk-WARNING **: 18:19:45.629: Could not load a pixbuf from /org/gtk/libgtk/icons/16x16/status/image-missing.png.
This may indicate that pixbuf loaders or the mime database could not be found.
**
Gtk:ERROR:../../../../gtk/gtkiconhelper.c:494:ensure_surface_for_gicon: assertion failed (error == NULL): Failed to load /org/gtk/libgtk/icons/16x16/status/image-missing.png: Unrecognized image file format (gdk-pixbuf-error-quark, 3)

It does require the developer to include shared-mime-info in their package includes.

[azubieta]

Do you tested that the resulting database cache is portable ?

[TheBrokenRail]

All the generated files seem to just be localizations and extension globs, it also works fine in a Docker container.

[azubieta]

Thanks for the contribution !