Editor: allow to display more view context rows before edit row

[ta2-1]

After #5786 is landed there will be the only (probs two) view context rows available.
This will be the price for fixing a jumpiness of edit row positioning.
So we will need to give users a possibility to show more rows at least or to come with something new.

[dwaynebailey]

Firstly, I don't think this is the final solutions but at least something we can do before anyresponsive design effort.

Suggested solutions:

1. Allow user to set the previous context row count in the same way that they can set unit retrieval.
   + easy to do, - not discoverable
2. Add icons to allow you to increase or decrease context rows. Add this to the unit location row.
   + its exposed and available, - requires some thought on how to merge this with the other context row implementation
3. Check viewport size and adjust the context row count based on some heuristic with some max such that the editor is in the centre.
   + no user setting anything, - we likely will get it wrong sometimes and can't expand if the user wants more context than the viewport allows
4. Make the header more compact as soon as you've moved off unit 1.
   + frees up more space for context units, - header changes, doesn't solve issue if units are large.