Add web proxy configuration support for linux clients network probes

Notes:
- Linux Java clients affected: insight, editor, importer
- Windows/MacOS clients not affected (network availability detected via network interface)
- UpgradeCheck behaviour affected on all clients behind a proxy:  failing to configure proxy settings will result in timeout on connection to http://upgrade.openmicroscopy.org.uk (and possibly skewed usage stats). Not adressed here.
- Client server compatibilty check kicks in before NetworkChecker - not sure whether this is the intended behaviour (should it be the other way around?)