Refactor decoding of clip data

Decoding of UMLObjects now uses the same code path on all clips. Fixes that
recently went to clip1 are now also active for clip2 (diagrams). List view
items are never decoded, but created in response of a signal of new UMLObjects.

When pasting clip2 in the same file, only the diagram gets duplicated and not
all contained UMLObjects, like described in feature request 85807.