* Button-1
  - position insertion cursor just before character under (mouse) cursor
  - set input focus to text widget
  - clear selection

* Button-1-motion
  - stroke out selection between insertion cursor and character under
    (mouse) cursor

* double-Button-1 
  - select word under (mouse) cursor
  - position insertion cursor at end of word

* double-Button-1 motion
  - stroke out selection consisting of whole words

* triple-Button-1
  - select line under (mouse) cursor
  - position insertion cursor at end of line

* triple-Button-1-motion
  - stroke out selection consisting of whole lines

Ends of selection can be adjusted by dragging with mouse button 1
while the Shift key is down.
This will adjust the end of the selection that was nearest to the
mouse cursor when button 1 was pressed
If the button is double-clicked before dragging then the selection
will be adjusted in units of whole words.
If it is triple-clicked then selection will be adjusted in units of
 whole lines.

* CTRL-Button-1
  - reposition insertion cursor without affecting selection

* Button-2
  - copy selection into the text at position of mouse cursor

* Button-2-motion
  - adjust text view
