Dynamic/plugged in handlers

[tcalmant]

iPOPO should support a new kind of handlers, which could be added to live instance managers at runtime.
There activation couid be requested in two ways:

• the component indicates that it accepts to be handled by an optional handler: if it's here, the handler is bound to the component, else the component is started as normal.
• the handler indicates the components it wants to be bound to.

The idea is to enable optional non-functional properties handling using iPOPO handlers.
For example, the logger sample handler could be enabled for all components as soon as its bundle is started, but it must not prevent the instantiation of components if it is absent.

As they can be enabled after the instantiation of the component, and event after its validation, those handlers should not be able to manipulate the component.