Add action to show a swing tree for selection.

This new action fires up the scalac tree browser on the
selected source in the current editor. This helps track
down position-related bugs, such as issues in hyperlinking,
hovers, completions, etc.

* also fixed 'race condition' errors in the spy view.

![screen shot 2013-06-02 at 2 18 04 pm](https://f.cloud.github.com/assets/133742/596289/a8b4f8e4-cb7e-11e2-8e4c-e040d7adf764.png)