package: add the ability to guess prefix and libdir

If no prefix/libdir is passed in the parameters, guess it
by looking at where the spawned JS file lives, and walking up
the hierarchy.
This assumes the application is configured following the FHS,
and is installed in $(pkglibdir), togheter with the src GResource.
It also assumes that $(pkglibdir) is something like $(prefix)/lib{32,64,}/$(name),
that is, exactly one level of directory to get from $(pkglibdir)
to $(libdir), and one level of directory from there to $(prefix).
This means that it is unsuitable for debian multi-arch, but this
is not really a problem: this is an application, it makes no
sense to multi-arch it.
Switch to the new package support for relocatable apps

Stop passing the prefix and libdir to package.start(), and let
it guess it from System.programInvocationName.
package: recognize if the application launched is a symlink

And if so, resolve the symlink before using the application name.
Use package.js from gjs

Replace our copy with the upstream one, and remove the hacks
we had to run with it.
Fix startup notification