Write duplicate attachments to the binary inner header only once and
skip duplicate entries when reading a KDBX4 file.
This fixes a an attachment mapping problem when an attachment appears
more than once in a database (which occurs frequently when editing attachment
entries and history is turned on)
* Adds KDBX4 reader/writer interfaces
* Adds KDBX4 XML reader/write interfaces
* Implements test cases for KDBX4
* Fully compatible with KeePass2
* Corrects minor issues with Argon2 KDF