corrected update line number widget widget connection

this slot didn't use the block count argument, so its argument list was
set to void; however, the argument type was not removed from the connect
call (causing a no such slot warning)
corrected problem with reporting lines with backspace

the issue was that when a new line was inserted (by control+return or
pasting a newline) and marked as an inserted line (modified type), if
backspace is then used to combine with the previous line, then the next
line is erroneously reported as deleted

to correct the problem, backspace now checks if the line that is about
to be reported as deleted has a line inserted modification type, and
then change the type back to line changed (the previous line will be
marked as modified once the cursor is moved there) and not emit the
line deleted signal

also, if the inserted (next) line being combined is blank, then the
previous line should not be marked as modified, so the ignore change
flag is set, but another problem was found - for some reason (for
delete and backspace) the document changed signal is sent twice,
defeating the ignore flag

therefore, instead of resetting the ignore change flag in the document
change slot when set, it is now the responsibility of the caller to
reset this flag (undo and redo were already doing this) after calling
whatever function that will trigger the document change signal