Execute encoding/decoding of clipboard data in background...
was seeing a lot of red flashes for large file selections.
Move the majority of the Clipboard management code into a
shared DocumentClipper class.
Hide the copy-to and move-to menus in same.
BUG=20915675
Change-Id: Iff59e846afe6f1e90e6be816f5860d8b7efe3ae1
bdbd0ffdf2