Moving the depth checker from NSXMLElementAdditions.m to be in DebuggerWindowController
* Source/NSXMLElementAdditions.m:
([NSXMLElement isLeaf]): Moved the checking to see if we need to fetch more
* Source/NSXMLElementAdditions.h: New header file for linking
* Source/DebuggerWindowController.m:
([DebuggerWindowController outlineViewItemDidExpand:]): New method that handles the depth checking
* MacGDBp.xcodeproj: Project changes for NSXMLElementAdditions.h