Invalid return for eInternalContainer

[gdaniel]

The return of eInternalContainer is not always consistent with EMF standard implementation.
Fixing it may cause important performance problems (need to be studied).

See this test case for examples (tests have been commented to avoid build failure)

[yvrng]

Fixed in #69 : The stores are updated according to the EMF calls, so that the backends are always synchronized. There is no longer custom processing during set() and add().