Centroid is a property, not a callable
Initial logic for adding a tree

* "Add Tree" button activates addTreeMode
* Set up click control and drag control in OpenLayers
* Logic for activate/click/save/close

Also misc tweaks:
* activateMode() does nothing if you're already in that mode
* modes other than "browse" initially hidden
Disable field entry until user clicks map

Also add simple markup for sidebar
Utility $find() complains if a referenced selector is missing