Improvements in DefaultImportationLinker

Use ServiceReference<ImportDeclaration> in place of ImportDeclaration to
store in DeclarationsManager.
The most of methods now manipulate ServiceReference<ImportDeclaration>.
Handle ImportDeclaration modification by unbind an bind the ImportDeclaration.

Some variables names changed and some methods return changed.
Remove the UniqueImportation property

Remove the UniqueImportation property of the DefaultImportationLinker.
Add JavaDoc to DefaultImportationLinker