Fixed: warning raised by the compiler due to unknown type in var declarations
Fixed: memory leak in CPComboBox

Previously, when assigning a listDelegate to a CPComboBox, we never deleted the observers added by this method. Now we add and delete these observers in the method addObservers and removeObservers.

This PR fix another issue. Previously, the behavior of the panel of the comboBox wasn't the same as the one in Cocoa. Now, when closing the panel, the hit view won't be the first responder as it was. Cocoa works like this as well.