Split screen for version comparison

[tomusdrw]

Related: #8

To simplify comparing differences between version, we should display two instances of GP one next to another in a vertical split mode (see #61).
Each part of the split screen would have different version selected.
Selection should be synchronized between these two instances (if the version is different).

User workflow:

1. User selects fetch host call on version 0.6.6 (or just comes from a link from the code).
2. The second version is 0.7.0 and the reader attempts to open the same selection (same mechanism that's in place where switching between versions manually).
3. The second screen is auto-scrolled close to the previous selection or the selection is just in-sync, so that user can compare (manually) the changes.