[Imagine plugin] Changed outset into outbound in plugin

Q | A
------------- | ---
Bug fix? | no
New feature? | no
BC breaks? | no
Deprecations? | no
Tests pass? | -
Fixed tickets |
Refs tickets | zikula@cc9c57b
License | MIT
Doc PR | -

In the Imagine plugin now also everywhere outbound is being used for THUMBNAIL_OUTBOUND mode instead of outset.
We have inset and outbound now.