Add support for Tree Style Tab

These two commits add support for opening tabs as child of the current one if [Tree Style Tab](https://addons.mozilla.org/de/firefox/addon/tree-style-tab/) is installed.

* The first commit add the basic support and can be merged alone. In this case, tabs are opened as child tabs if the position setting is set to *right*.
* The second commit adds a new option to explicitly enable the behaviour. (*right* stays the default)

The zh-CN locale is not included because I do not speak Chinese.