Allow jalview auto-updates to download to and work from separate user-space directory

As described in JAL-3620,
When Jalview is installed in system-space for system-wide use by (likely) multiple users, the installation appdir will not be writable by the user, and so Over-The-Air automatic updates (by getdown) will not be possible and will fail to happen.
Comment https://issues.jalview.org/browse/JAL-3620?focusedCommentId=23589&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-23589
describes having a parallel appdir in user space (e.g.
~/.jalview/updates/ or equivalent) where updated files can be downloaded if the installation appdir is not writable.

Install4j and getdown will need to know to look in this alternative location for possibly more up-to-date jars (install4j will only need to look for an updated getdown-launcher.jar). There should be a jar version check (which means jar versions will need to be added to the jar manifest) as presence of a jar in the user-space location does not guarantee it is newer (e.g. there might be an updated jar from a previous system installation version than the current system installation).

Install4j allows code execution in the installer (although such code addition hasn't been used by jalview yet) so this should be possible (to alter the classpath accordingly before running getdown-launcher.jar).

The option to use this user-space updates feature or to just use the system installed version could be set in ~/.jalview_properties.