Behavior after closing tab with children

[hrj]

There are three possibilities

1. Promote the first child to be the parent. This is the current behavior and matches that of TreeStyleTabs. However, it is a little weird, because siblings of the first child tab suddenly become its children.
2. Promote the whole sub-tree of the tab one level up. This is very intiuitive but when the closed tab is a root tab, it will suddenly create a number of new root tabs. Hence, the next alternative is:
3. Use (2) except when the closed tab is root. In that case use (1).

(3) makes the most sense from a usability point of view.

However, we should see how this interacts with #3