Allow to call appimagetool from the outside

[probonopd]

Currently we have to do

./linuxdeployqt*.AppImage --appimage-extract
PATH=./squashfs-root/usr/bin:$PATH ./squashfs-root/usr/bin/appimagetool ./appdir/

Maybe we can use a more elegant solution, like

ln -s ./linuxdeployqt*.AppImage appimagetool
./appimagetool ./appdir/

Or even just

./linuxdeployqt*.AppImage appimagetool ./appdir/

But low priority since not that big an improvement.