[e4] EclipseContext.get(Class) should create creaatable classes

[laeubi]

Currently if I call EclipseContext.get(SomeCreatableClass.class) it returns null, but ContextInjectionFactory.make(SomeCreatableClass.class, context) returns a value, also @Inject works if the class is marked with org.eclipse.e4.core.di.annotations.Creatable.

It would therefore be good if the EclipseContext.get check if the class is creatable and the create an instance if required, alternatively we can have a getOrCreate(Class) for that purpose.