Allow delegate to alter panners configuration while a panner is active.

"high-level" internal API for panners used to be removePanners and addPanners. New method setNeedsAddPanners should be preferred instead.

setNeedsAddPanners immediately replaces panners if no panner is active, or wait for the last active panner to complete before replacing them.

The consequence is that delegate methods that are fired during a panning touch can now safely change panner-related configuration, such as panningMode.

Before, those configuration change would invalidate an active panner, and prevent IIViewDeckController to perform panner completion cleanup code.