Change EventDispatcher field visibility of AbstractHandler to protected.

Change EventDispatcher field visibility of AbstractHandler to protected.

Instead of having EventDisptacher private, we set it as protected thus
all the classes that extends AbstractHandler can use the variable.
This patch remove:
- the setDispatcher() method from AbstractHandler
- the declared EventDispactcher field of the classes extending AbstractHandler

Change-Id: Idbd000def09aa91cc4e40976ac4a37274bfce762
Signed-off-by: Alexis de Talhouët <adetalhouet@inocybe.com>