What software can I use to make "stop motion" video from existing video material I want to make video like this: 
                
            

While I know they used different technique (stop motion from photos), I would want to make mine from existing video of person walking. So my question is: what software would automatically remove every frame except the ones I mark as important for walk animation (ie.: frames in which left or right leg is in specific position so the walk impression is preserved).

Even better, the video I currently have has pretty systematic walk so I guess it would be nice if I could specify the important frames as x+n and y+n (where x and y are numbers of first important frame for left and for right leg and n is some constant offset) and just adjust the n if and when necessery.

Even better if the software cold actually detect which frames may be important.

So, can this be done and if yes then what software do I need?
You can use any non-linear video editor to make this, but you are in for a lot of manual work.

Time posterizing (or video strobe) is mentioned which can give you that "stop" effect, but it will not take care of the walk-cycle. For that you need to manually go in and choose each frame.

My suggestion would be to take the video sequences you need, go through frame by frame in the preview monitor and grab the frame you need according to where in the cycle by making the in and out point the same, then drag the cut (a single frame) onto the timeline. Repeat.

Now the only thing you need to do is to scale the timeline so that each image has a duration longer than 1 frame (probably around 10 frames or so). With this technique you can also fine-adjust a single "take" if needed.

The other approach is to convert video (or using photo still) as single images. Here you loose the advantage of fine-adjusting though (in relation to the walk-cycle).

Automatically detecting where in a walk-cycle a person is is highly specific task and AFAIK there exist no such software available to the public (some law enforcement agencies have access to software for behavioral analysis including walk patterns, but that becomes quite a different table- - so yes, it's possible, but not available).