Improve PagePainter and LineAnnotPainter documentation

Write doxygen documentation for:
* PagePainter::paintPageOnPainter(), paintCroppedPageOnPainter()
* `Change Colors` algorithm functions of PagePainter
* PagePainter::drawShapeOnImage(), drawPixmapOnImage(); very straightforward,
  but added a note on the coordinate system.
* LineAnnotPainter constructor, draw()

Remove unused functions:
* PagePainter::cropPixmapOnImage(), QPixmap::copy() can do it easily today.