Improvement regarding tabs overflow

[David Nolden]

I think we have enough document lists:
- CTRL+Tab
- Working-set tooltip
- Documents tab
- Quickopen list

The scroll-buttons do suck indeed. To solve the problem, the first
priority should be wasting less space (as Milian suggested). There are
huge margins around the text, for example.

Another possibility would be stacking multiple tab-bars above each
other if there are too many tabs. After all, the only sense of a
tab-bar is seeing all open documents at the same time.. some grouping
would be nice too.