Add callbacks to panels before and after becoming active/inactive

Added a protocol for panels that contains four optional methods:

    - (void)willBecomeActiveAsPanelAnimated:(BOOL)animated withBounce:(BOOL)withBounce;
    - (void)didBecomeActiveAsPanelAnimated:(BOOL)animated withBounce:(BOOL)withBounce;
    - (void)willResignActiveAsPanelAnimated:(BOOL)animated withBounce:(BOOL)withBounce;
    - (void)didResignActiveAsPanelAnimated:(BOOL)animated withBounce:(BOOL)withBounce;

These are called (if the panel has implementation) in `_showRightPanel:bounce:`, `_showLeftPanel:bounce:` and `_showCenterPanel:bounce:`. This enables the side panel to be notified when the state changes.