first trial for MarkerVLine

this is just a test to see how the pull request and everything works.
the plugin build but doesn't show up in the "Marker" dialog, so it is completely untested.
I used only a copy-try-and-error approach, so there will be a lot of nonsense. But nonetheless any comments are welcome as it is the fastest way to get familiar with the program.

anyway, it would be good if you have a look at the request and can provide a new version or something similar that I can test the merging of your master branch to my branches.