Add Pane::onWillRemoveItem event.

src/pane.coffee

@@ -163,6 +163,15 @@ class Pane extends Model
   onDidRemoveItem: (callback) ->
     @emitter.on 'did-remove-item', callback
 
+  # Public: Invoke the given callback before an item is removed from the pane.
+  #
+  # * `callback` {Function} to be called with when items are removed.
+  #   * `event` {Object} with the following keys:
+  #     * `item` The pane item to be removed.
+  #     * `index` {Number} indicating where the item is located.
+  onWillRemoveItem: (callback) ->
+    @emitter.on 'will-remove-item', callback
+
   # Public: Invoke the given callback when an item is moved within the pane.
   #
   # * `callback` {Function} to be called with when items are moved.
@@ -358,6 +367,8 @@ class Pane extends Model
     index = @items.indexOf(item)
     return if index is -1
 
+    @emitter.emit 'will-remove-item', {item, index, destroyed}
+
     if Grim.includeDeprecatedAPIs and typeof item.on is 'function'
       @unsubscribe item
     @unsubscribeFromItem(item)