Default getdown/install4j build should not reference genuine appbase so tests can be done on new jar files

For a test compile and build, launching Jalview using the getdown launcher (either directly or with an install4j installer) will immediately download whichever version of Jalview was last on the TEST channel website. This is likely to not be the version you just compiled.

This could be fixed by defaulting to a file:// URL for appbase, and putting the jars in a separate folder for "downloading" into the right place [note this means that you will end up with two copies of the jars in the Jalview folder].

An immediate fix would be to have an appbase that doesn't work and only use the OFFLINE installers (so that the installer includes all the jars).