Add gnome-icon-theme to the bundle

[aferrero2707]

The lack of the gnome-icon-theme on the host system can lead to crashes (see #54, #53 and this paragraph in the GIMP wiki).

For better portability the icons should be included in the bundle, as this should not increase the size of the AppImage too much.

[sudo-give-me-coffee]

Since "image-missing" is the last indentation image, theoretically you just need to put an empty svg icon in usr/share/hicolor/status/image-missing.svg and also, bundle the index.theme of hicolor icon theme.

It is unnecessary to embed an entire package icon theme, because most icons will never be used