Edit Book: Allow editing xpgt (Adobe Page Template) files as XML.

Fixes #1346543 [Edit Book: Allow editing xpgt files](https://bugs.launchpad.net/calibre/+bug/1346543)
Edit Book: Make the undo/redo/cut/copy/paste buttons on the editor toolbar also configurable, so they can be removed via Preferences->Toolbars, if needed.

See #1346913 (Edit Books - Toolbars)
Edit Book: When re-arranging the toolbars inside an individual file editor, save the new toolbar layout so that it is re-used in the future.  Also apply the changed layout to all open editors of the same type. See #1346913 (Edit Books - Toolbars)