List of additions/changes
    1. EditorWindow uses LineNumber.Text instead of tkinter.Text.
    2. Added linenumber canvas to IDLE windows except PyShell
    3. Some info about LineNumber.Text
        a) Inherits tk.Text
        b) Generates <<Changed>> virtual event, when insert, delete, replace,
           cursor changes position, window resized, scrolled etc. Nothing else
           is affected. The result of the original call is returned.
    4. LineNumber.LineNumberCanvas info
        a) font_color and breakpoint_color have default values
        b) Linenumber and breakpoints disabled by default. Instantiating
           a LineNumberCanvas object does not pack it.
        c) Pack it programmatically by calling its "attach" method.
           Compulsorily supply the text widget to attach to.
           Supply other parameters like font_color, background, 
           breakpoint_color as necessary.
        d) Unpack using "detach" method.
        e) Breakpoint feature is enabled only if LineNumberCanvas can 
           "see" an EditorWindow instance called "editwin" as its attribute.
           It(editwin) should implement set_breakpoint(linenumber) and 
           clear_breakpoint(linenumber) methods. EditorWindow responsible
           for binding breakpoint action on the canvas to LineNumberCanvas'
           toggle_breakpoint method.
        f) Contains a htest for GUI testing linenumbering and breakpoints.
        g) Any valid color can be set for linenumber canvas' background,
           its font color and breakpoint color. NS: Breakpoint does not have
           a background color.
        h) Linenumber canvas enabled by default.
    5. Linenumber preferences in configDialog's "General" tab.
       Highlight preferences in  configDialog's "Highlight" tab.
    6. Other changes
       a) Refactoring of PyShell's set_breakpoint_here, set_breakpoint,
          clear_breakpoint_here and clear_breakpoint to make more consistent.