Bundle appimagetool properly

Looks like `appimagetool` nowadays has dependencies that also need to be deployed.

@TheAssassin says we should do something like

* https://github.com/AppImage/AppImageKit/blame/master/ci/build-appdir.sh#L61-L71
* https://github.com/linuxdeploy/linuxdeploy-plugin-appimage/blob/master/ci/build-appimage.sh#L50-L58

Needed for:
* https://github.com/probonopd/linuxdeployqt/pull/541