Add Tree Style Tab support for opening new tabs

Add Tree Style Tab support for opening new tabs

Since commit 05d60dba5 we no longer open links in new tabs by simulating
ctrl-clicks on them, but instead use Firefox APIs. Before, the new tabs were
detected by the Tree Style Tab extension and opened as new children of the
current tab. After that commit new tabs opened as new _siblings_ after the
current tab instead. This commit opens new tabs as children again, by using the
public [Tree Style Tab API][tst-api].

[tst-api]: http://piro.sakura.ne.jp/xul/_treestyletab.html.en#api.