open and save .hocr files

[milahu]

fix #730

also add a CLI arg parser with a pretty helptext

helptext

$ gimagereader-qt6 --help
Usage: gimagereader-qt6 [options] [files...]
A GTK/Qt frontend to tesseract OCR

Options:
  -h, --help     Displays help on commandline options.
  --help-all     Displays help, including generic Qt options.
  -v, --version  Displays version information.

Arguments:
  files          Files to open, optionally. These can be image files or hocr
                 files. Every image file is seen as one page. Hocr files can
                 reference image files for pages or graphics.

$ gimagereader-qt6 --version
gImageReader 3.4.3 (d00b33)

todo: open and save .hocr files with the GTK version

[manisandro]

Thanks, Note that there are many other locations in the code where html is the expected extension for hOCR files.

[milahu]

many other locations

yepp, i tried to fix some more

maybe we should add an option for the default output hocr file extension
stored in ~/.config/gImageReader/gImageReader.conf
because some users may not be happy with .hocr by default

[milahu]

needs refactoring