Changes

CompletionContext should check the editor if we are in a workspace or
not. fixes #4686
(the setter of CompletionContext will be cleaned up more when we remove
the old completion)

nil check if editor is not set

fix type inference for "self", so now code completion knows that "self"
is a SmallInteger when inspecting "1"

lookupDefiningContextFor: startingFrom:  was using variable names. It is
better to instead walk up contexts till the scope is the definition
scope of the variable (independend of the name).

forgot to remove "*not*"