[CAS-14798] update AppImage creation and uncouple build from 'casa-source'
The uncoupling of the 'casa-source' tree from the build is important. Previously,
the 'xml-casa` script within 'casatools' was used for generating binding code.
This seemed like a win when the 'casa-source' tree was always updated as part of
any update. Now, however, with the C++ GUIs in longterm maintenance mode, it is
more important to be able to leave 'casa-source' unmodified while updating the
build system.
