Don't call slideDetailDownAnimated: on init

This was causing problems with other UI elements
(the GPS button) which move relative to the detail
view.
Don't attempt to hide detail if already hidden

This is a noon anyway, and complicated the process
of setting up views that move relative to the
detail view.
Clear the selection when tapping an empty spot

Fixes #119
Merge pull request #136 from jwalgran/topic/tree-selection

Tree selection / detail panel updates