EditorWindow.py:

+ Added get_tabwidth & set_tabwidth "virtual text" methods, that get/set the
widget's view of what a tab means.

+ Moved TK_TABWIDTH_DEFAULT here from AutoIndent.

+ Renamed Mark's get_selection_index to get_selection_indices (sorry, Mark,
but the name was plain wrong <wink>).

FormatParagraph.py:  renamed use of get_selection_index.

AutoIndent.py:

+ Moved TK_TABWIDTH_DEFAULT to EditorWindow.

+ Rewrote set_indentation_params to use new VTW get/set_tabwidth methods.

+ Changed smart_backspace_event to delete whitespace back to closest
preceding virtual tab stop or real character (note that this may require
inserting characters if backspacing over a tab!).

+ Nuked almost references to the selection tag, in favor of using
get_selection_indices.  The sole exception is in set_region, for which no
"set_selection" abstraction has yet been agreed upon.

+ Had too much fun using the spiffy new features of the format-paragraph
cmd.