support <CNTRL/T> immediate editing command only in string parameters

 * fixes autocompletion at the beginning of string params
 * is also available in filename commands and would allow filename completion after string beginning
 * also did some editing command cleanup
support <CTRL/W> immediate editing command: depending on context, rub out words or entire commands

 * when rubbing out words (in string params), use Scintilla's definition of a word
States::is_string() to check whether current state is a string-state
fixup: word-rubout may rub out nothing
<CTL/U> immediate editing command to rubout current string param

when not in string-state, <CTL/U> is self inserting (unlike traditional
TECO where it would clear your entire commandline)