Starting to implement the source viewer pane

Starting to implement the source viewer pane

* Source/DebuggerWindowController.h: Adding outlets for StackController, _currentFile, and _sourceViewer
* Source/DebuggerWindowController.m:
([DebuggerWindowController tableViewSelectionDidChange:]): Added delegate notification for when the selection of the tableview changes to update the source viewer
([DebuggerWindowController updateSourceViewer]): New method to refresh the source viewer display
* English.lproj/Debugger.nib: Hooking up the new outlets