Changed c4dtools.resource.load()

- c4dtools.resource.load() now returns a tuple. The first item is the
dictionary returned in previous versions of c4dtools. The second is a
boolean value. This value is only True when a cached symbol-file exists
and the original resource symbols have been modified.
- c4dtools.resource.Resource class has now a `changed` attribute. It is
set in c4dtools.prepare() on the return-value of load().
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