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
public void customButtonPressed(DialogField field, int index) {
			sourcePageCustomButtonPressed(field, index);
		}
		
		public void selectionChanged(DialogField field) {}
		
		
		public void dialogFieldChanged(DialogField field) {
			sourcePageDialogFieldChanged(field);
		}
	}
	
	private void sourcePageCustomButtonPressed(DialogField field, int index) {
		if (field == fFoldersList) {
			List elementsToAdd= new ArrayList(10);
			switch (index) {
			case 0: 
				CPListElement srcentry= createNewSourceContainer();
				if (srcentry != null) {
					elementsToAdd.add(srcentry);
				}
				break;
			case 1: 
				CPListElement[] srcentries= chooseSourceContainers();
				if (srcentries != null) {
					for (int i= 0; i < srcentries.length; i++) {