Add a bunch of new NSView classes (to form a meta-view BSSourceView) that adds a...

Add a bunch of new NSView classes (to form a meta-view BSSourceView) that adds a line-numbered NSTextView embedded in a NSScrollView

* English.lproj/Debugger.xib: Use our BSSourceView instead of an NSTextView for the source viewer
* Source/BSLineNumberView.h: new file
* Source/BSLineNumberView.m: new file
* Source/BSSourceView.h: new file
* Source/BSSourceView.m: new file
* Source/BSSourceViewTextView.h: new file
* Source/BSSourceViewTextView.m: new file
* MacGDBp.xcodeproj: Project updates for all the new files
* Source/DebuggerWindowController.h: Switch to using a BSSourceView
* Source/DebuggerWindowController.m: ditto