Added undo funtionality in 'code frame'

First branch (undo_redo) commit:
The file gui/internalframes/concreteframes/editor/EditorFrame.java
was changed to make use of the new functionality within the 'coding frame'
window.
Merge branch 'undo_redo' of https://github.com/agarbade/openDLX into agarbade-undo_redo
Added CTRL+Z and CTRL+SHIFT+Z for undo/redo

Added shortcuts for undo and redo. Also deleted some comments.
Added menu for undo/redo

Also refactored instantiation of undo manager.
Added toolbar instead of buttons

Also added icons from the tango project for the toolbar
Updated contributors
Capitalisation of menu entries and descriptions