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.
Merge pull request #428 from dragos/spy-improvements

Add action to show a swing tree for selection.