Send views header, footer, etc. for translation on entry

Currently translation of views header, footer, title, and empty strings are done on the fly by calling tt() with only the first two arguments. This more or less works, but means we don't get update functionality. When a header string is changed, we'll get a new string added, with the existing one orphaned.

Instead, here as elsewhere we should be calling tt() with the $update arg set to TRUE when items are added or updated.

Attached patch does this by altering the relevant Views form and attaching a submit handler.