Intelligently activate new items when the active item is closed

If the active item was the pane's first item, the second item (which is
the new first item) is activated. Otherwise, the item to the left of the
closed item is activated.

src/pane.coffee

@@ -158,8 +158,10 @@ class Pane extends Model
     if item is @activeItem
       if @items.length is 1
         @activeItem = undefined
-      else
+      else if index is 0
         @activateNextItem()
+      else
+        @activatePreviousItem()
     @items.splice(index, 1)
     @emit 'item-removed', item, index, destroying
     @container?.itemDestroyed(item) if destroying