UNDO added
Added a multilevel UNDO (Ctrl-Z) for most functions that change the content. Other changes:
- Backspace at the first non-blank character mimics BackTab, if Auto-indent is enabled
- Added a REDRAW (Ctrl-E) function, which checks for the changed screen size after the window size has been changed.
- Added a line number column on the left side of the screen (can be turned off).
- Improved the scrolling speed, such that it lags less.
- Some code simplification and straightening, such that functions group better and a easier to understand.