JBIDE-13987 : properly load proxy wizards from remote sources

Signed-off-by: Fred Bricon <fbricon@gmail.com>
JBIDE-14663 : don't schedule file download twice

also skips source lookup for maven projects

Signed-off-by: Fred Bricon <fbricon@gmail.com>
JBIDE-14663 : catch CoreException instead of Exception

Signed-off-by: Fred Bricon <fbricon@gmail.com>