Currently there is an event published for every disposal of an
IEclipseContext to IEclipseContext.TOPIC_DISPOSE
but because the properties are completely empty one needs to guess if
the context one was using is actually affected. Even worse, it is not
actually possible to know if a context is/was disposed or not.

The event now contain the disposed IEclipseContext so it is more generic
e.g. if one want to dispose resources from a context function.

Fix https://github.com/eclipse-platform/eclipse.platform.ui/issues/215