Improvements in "Show Invisible Characters"

* Spaces (U+20) are also displayed as '.' when showing invisible characters
* No representation for space characters when hiding invisible characters, except for U+FEFF which is deprecated as a space character
* Show a marker for special characters when showing invisible characters
* Changing representation of NO-BREAK SPACE from '.' to '~', to avoid confusion on showing invisible characters

(Related to http://lists.macromates.com/textmate/2014-March/037194.html)
(public domain patch!)