Bug 4240 Can't edit class path entry (1GKEPIF)
There is no way to edit an existing entry for a library on the Java Build Path project properties page (other than editing the .classpath file directly). NOTES: EG (9/22/2001 11:42:11 AM) this is particularly annoying for variables. we should have an Edit button when a variable is selected.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/SourceContainerWorkbookPage.java
if (cpe.getEntryKind() == IClasspathEntry.CPE_SOURCE) {
				if (fProjPath.equals(cpe.getPath())) {
					srcelements.clear();
					
					fProjectCPEntry= cpe;
					fIsProjSelected= true;
					break;
				} else {
					srcelements.add(cpe);
				}
			}
		}
		fFoldersList.setElements(srcelements);
		
		
		fFolderRadioButton.setSelection(!fIsProjSelected);
		fProjectRadioButton.setSelection(fIsProjSelected);
	}			
	
	public Control getControl(Composite parent) {
		PixelConverter converter= new PixelConverter(parent);
		
		Composite composite= new Composite(parent, SWT.NONE);
		
		GridLayout layout= new GridLayout();
		layout.numColumns= 2;		
		composite.setLayout(layout);
		
		fProjectRadioButton.doFillIntoGrid(composite, 2);
		fFolderRadioButton.doFillIntoGrid(composite, 2);