Evas filters: Fix proxy usage (source unset)

Evas filters: Fix proxy usage (source unset)

Proxy sources & objects were not properly unset.
This results either in crashes (especially in the Edje tests)
or dangling objects with tons of references.

Remove the refcount increase/decrease, as it is redundant.
Store pairs proxy+source instead of just the source in all hashes,
so we can unset the is_proxy flag on the proxy when there are no
sources anymore.