QTextLayout selections doesn't work correctly with trailing spaces

When drawing a line contains only spaces and has any selections using QTextLayout::draw(), no selection is shown.

To solve this problem, I've corrected QTextLayout and QTextEngine as following.

• QTextLayout.cpp / line 1088
  original code : int sl_length = sl.length + (isLastLineInBlock? 1 : 0); // the infamous newline
  modified code : int sl_length = sl.length + sl.trailingSpaces + (isLastLineInBlock? 1 : 0); // the infamous newline

• QTextLayout.cpp / line 1097
  original : if (sl.length && (selectionStartInLine || selectionEndInLine)) {
  modified : if (sl_length && (selectionStartInLine || selectionEndInLine)) {

• QTextEngine.cpp / line 3484
  original : lineEnd(line.from + line.length),
  modified : lineEnd(line.from + line.length + line.trailingSpaces),

Of course I know I may not consider all cases, but I think that above codes can help error correction.