Initial breakpoint adding code

Initial breakpoint adding code

* MacGDBp.xcodeproj: Project updates for Breakpoint.m/h
* Source/AppDelegate.h: Add a breakpoints ivar
* Source/AppDelegate.m:
([AppDelegate addBreakpoint:]): New method
([AppDelegate breakpointsForFile:]): New method
* Source/BSLineNumberView.h: Add a markers ivar
* Source/BSLineNumberView.m:
([BSLineNumberView drawRect:]): Add breakpoint-detecting code
* Source/DebuggerWindowController.m: Making this the delegate of BSSourceView
([DebuggerWindowController gutterClickedAtLine:forFile:]): Implementing delegate method