We now properly handle the mouseDown event in the gutter

We now properly handle the mouseDown event in the gutter

* Source/BSLineNumberView.h: New property called lineNumberRange
* Source/BSLineNumberView.m: Synthesize lineNumberRange and set it to be empty in -[init]
([BSLineNumberView drawRect:]): Set lineNumberRange to be the range of visible lines in the source viewer
([BSLineNumberView mouseDown:]): Use the line number range to simplify the code of determining which line was clicked
* Source/BSSourceView.h: New property file to hold the filename; also create delegate category
* Source/BSSourceView.h: Synthesize file
([BSSourceView setFile:]): Sets the file property as well as setting the string of the textView object
* Source/DebuggerWindowController.m:
([DebuggerWindowController updateSourceViewer]): Use the new -[setFile:] method