Fix problem with metadata options appearing multiple times
Avoid loading .mkv attachments into RAM

Instead we keep a reference to where in the MKV file the attachment
is located and we can later open a slice of the mkv-file as the attachment.

This combined with the fact that freetype can open fonts from a file
handle reduces the static memory use for attachments to 0 bytes

Fixes #1689
Fix bug making 'Refresh metdata...' option in item menu not to work at all