Add in more exercises

[Mathnerd314]

See https://issues.workrave.org/show_bug.cgi?id=474. I also uncommented the existing exercises.

I'd like support for multiple exercise files, so e.g. we could have a base/good/bad/ugly split like gst-plugins, something simple like just loading all xml files in the directory, but I don't know the codebase that well...

[xeruf]

@rcaelers anything wrong here? :)

[rcaelers]

It seems I never explained why I didn't merge this pull request. My apologies.

These new exercises lack images that illustrate what you have to do.
Our current exercises do have images, but we occasionally get complaints on the female character in these images.
I planned to learn blender (tool used to create current images) and create new images for the existing exercises and for the exercises in this pull request; but never gotten to it...

Also, when we get more exercises, I think it should be possible to select which exercises Workrave will show.

It seems there is a new pull request with exercises (#292)

Finally, new link to archived bug report from bugzilla: https://workrave.org/bugzilla/474/

[Mathnerd314]

The images are definitely a sticking point. From a contribution standpoint I think the easiest thing would be to accept self-shot mobile videos. There's open-source tools that remove the video background. Shooting in a parking lot etc. with better lighting should remove the graphical issues that show up in my video (I did it in a crowded bedroom with the blinds closed).

stretch.mp4