on windows webengine seems to install a permanent ctrl+z handler that is
active even when an editing area is not selected, so our shortcut is
never triggered when the webengineview is focused.
- always store media references in fields in NFC form
- always encode filenames on disk in NFC form on machines other than macs
- use relevant encoding when placing files in the media folder during syncs
and apkg imports as well
- rename 'unused media' back to 'check media'
- check media can now automatically change media references and filename
encodings to the correct format
