Follow-up from "Clean up xdg-foreign v2 wrappers"

The following discussion from !203 (merged) should be addressed:

• @vladz started a discussion: (+1 comment)

  A couple more issues to address:

  • an imported resource should be created even if there is no specified exported surface with the provided handle, it's not a protocol error
  • xdg_imported_v2.destroyed event is sent to wrong resource in zxdg_importer_v2_import_toplevel