Clean up and simplify line editing functions.
Clean up and simplify line editing functions.

This greatly simplifies all line editing functions - especially
getstring() and showstring(). showcursor() is removed and integrated
into showstring(). del_char() and add_char() are simplified as well.
add_char() is renamed to ins_char().

Signed-off-by: Lukas Fleischer <af90e31281d8305af43dc616c5aaca3c7165f8a7@cryptocrack.de>

dunecn/calcurse,lfos/calcurse,lfos/calcurse,dunecn/calcurse,lfos/calcurse