[feature/plupload/integration] Integration of Plupload

This commit is a highly-refactored and up-to-date version of Fyorl's work
which was part of his Google Summer of Code 2012 project "Attachment
Improvements".

PHPBB3-10929

phpBB/styles/prosilver/template/overall_footer.html

@@ -65,6 +65,7 @@
 <!-- EVENT overall_footer_after -->
 
 {$SCRIPTS}
+<!-- IF S_PLUPLOAD --><!-- INCLUDE plupload.html --><!-- ENDIF -->
 
 </body>
 </html>