Edit Book: Fix various problems when editing EPUB files with non-ascii filenames on OS X. Fixes #1317883 [calibre should care of 'decomposed UTF-8' filenames on Darwin platform](https://bugs.launchpad.net/calibre/+bug/1317883)
Edit Book: When editing HTML files that have charset encoding declarations, automatically change the declared encoding (if any) to UTF-8 on save, since the editor always saves files in the UTF-8 encoding.

This prevents a mismatch between the declared encoding and the actual
encoding if the HTML file was originally in an encoding other than
UTF-8.