[ticket/12560] Add methods to set upload and temp paths in plupload

The paths will be reset to the default paths after an upload is complete.

PHPBB3-12560
Merge pull request #2766 from marc1706/ticket/12560

[ticket/12560] Add methods to set upload and temp paths in plupload
Merge branch 'develop-ascraeus' into develop

# By Marc Alexander
# Via Dhruv Goel (1) and Marc Alexander (1)
* develop-ascraeus:
  [ticket/12560] Add methods to set upload and temp paths in plupload