Add a screenshot timeline to the timelines panel. Screenshots are captured automatically every second during recording.

[madonk]

Closes #399.

[burg]

This is good work. I think you should try to make screenshots a new type of profile instead, because the current approach doesn't fit well with existing timeline data gathering mechanism (which is pushed entirely by the backend). Adding a new profile type is not much work, and the calls to the backend and UI views can be easily reused. Plus, then we don't need to add a new timeline and fiddle with the styles (they are going to be changed soon anyway).

[xeenon]

FWIW, we are removing the concept of Profiles soon. Everything will be a Timeline. I think a Timeline makes sense for this, considering we will be a few optional Timelines that you can add or remove — this could be one.