Allow linux client scripts to be run from a different directory

Download and extract OMERO.clients-XXX-linux.zip

The following should work on Linux and Mac OS X:
```
./OMEROinsight_unix.sh
cd /some/other/directory
/path/to/extracted/clients/OMEROinsight_unix.sh
```
The following should work on Linux:
```
ln -s /path/to/extracted/clients/OMEROinsight_unix.sh /usr/local/bin/insight
# (or some other directory in your PATH)
insight
# should run Insight
```
and similarly for `OMEROeditor_unix.sh` and `OMEROimporter_unix.sh`

Double clicking is distribution dependent, e.g. in Fedora a dialog pops up asking whether you want to run or edit it.
INSTALL.txt also updated.

See https://trac.openmicroscopy.org.uk/ome/ticket/11571