[VIM] Improved indentation handling

 - Made normal-mode indentation consistent with insert-mode indentation;
 - Improved 'Backspace' behaviour so that it deletes the same number
of spaces that 'Tab' adds;
 - Changed characters used for displaying whitespace if 'set list'
were to be activated.
[VIM] Swapped 'colon' and 'semicolon'

Done because ':' is used much more frequently, and ';' necessitates
one less keystroke.