JBIDE-15151 : gracefully handle uninitialized classpath containers

Signed-off-by: Fred Bricon <fbricon@gmail.com>
JBIDE-11319: Url for Eclipse-SourceReferences
JBIDE-15350 Using JBoss Tools Properties Editor, breaks the format

Implemented algorithm that compares loaded and current values and
if the value is modified, merges it in the way that preserves
format as much as possible.