ctf: Introduce IEventDefinition

This interface is extracted from EventDefinition. It allows all of
IEventDeclaration to return only the interface and leave the internals
in the internal folder.

This should improve the maintainability of the CTF plugin without impacting
performance or testability.

Change-Id: I398c4440c67738eaa80a2e63e6d562352d0d64a8
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/64412
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>