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.
2 jobs for master in 18 minutes and 30 seconds (queued for 2 minutes and 33 seconds)
Status Job ID Name Coverage
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passed #60192
clang_format

00:02:31

passed #60191
linux kf5-qt5 qt5.12

00:18:30

44.0%