imagej-omero: use the OMERO-5.0 update site

Now when we install ImageJ, we simply enable the OMERO-5.0 update site
(as well as the Fiji update site), which contains all the necessary
dependencies at the necessary and tested versions.
imagej-omero: move subroutines to the top
imagej-omero: use a more robust test for OMERO

We now look for blitz.jar, rather than only a directory called "lib".
imagej-omero: pass OMERO home as script argument

This is better than requiring OMERO_HOME to be set, since setting
OMERO_HOME has other (probably undesirable) ramifications.
imagej-omero: add TODO reminder about IMAGEJ_HOME
Rename imagej-omero to install-imagej

The script is migrating toward more of a focus on simply bootstrapping
an ImageJ installation with OMERO-5.0 update site enabled.
Make script generation script a shell script

Now that our ImageJ installation has everything included within it to
communicate with OMERO, we can generate scripts using a shell script
that simply invokes the ImageJ launcher with the script generator as
main class.
install-imagej: make ImageJ folder configurable
ScriptGenerator: use base OMERO dir as arg

Previously, the script generator accepted the exact path (e.g.,
$OMERO_DIR/lib/scripts/imagej where OMERO script stubs were supposed
to go). But it makes more sense to accept the root directory of the
OMERO installation, verify that the argument is in fact valid, and
then generate the scripts in the appropriate subdirectory always.

With this change, the generator now exits with a non-zero error code
if something goes wrong.
install-imagej: mark script executable
install-imagej: tweak indentation
install-imagej: simplify OMERO existence check
install-imagej: tweak usage instructions

The "--all" flag can be used to generate OMERO scripts
for *all* ImageJ modules, not just headless ones.
install-imagej: tweak section header
install-imagej: be more verbose about IMAGEJ_HOME

Otherwise, the user might be mystified why a particular directory
is being selected for installation.
install-imagej: lean on the gen-scripts script

This script will be (hopefully) distributed as part of the OMERO-5.0
update site, so will be present after ImageJ is bootstrapped. Let's
just use it directly, instead of replicating any logic in this script.
gen-scripts: add TODO reminder
Remove obsolete launcher scripts

We definitely do not want to be wrapping the "jython" executable with
our own hacky shell script. And the "imagej" launcher is no longer
necessary now that we are distributing all required OMERO JARs with the
OMERO-5.0 update site, and no longer need to add JARs from the actual
OMERO installation's lib/server directory to the classpath.
install-imagej: fix references to installer script
gen-scripts: always use $0/.. for ImageJ directory

This is because the gen-scripts script is expected to be distributed via
the OMERO update sites, and thus live in the lib directory of the ImageJ
installation.