[Qt] add messages when handling local payment request files

- important for the open URI dialog to give users feedback
  when a file is invalid etc.
Store orphan blocks in serialized form
qt: Minor message updates

Used sending/receiving addresses -> Sending/receiving addresses
Previously requested payments -> Requested payments (idea from @cozz in #3521)
Merge pull request #3374

bd70562 [Qt] add messages when handling local payment request files (Philip Kaufmann)
Merge pull request #3528

66a8829 qt: Minor message updates (Wladimir J. van der Laan)
qt: English translation update
Merge pull request #3532

f087ce8 qt: English translation update (Wladimir J. van der Laan)
Merge pull request #3516 from sipa/serorphans-head

Store orphan blocks in serialized form