As discussed in #4317, the next KeePass2 release will ship with
support for a new generation of XML key files which enable
hash integrity checks.
This patch adds support for reading and generating this new format.
By default, KeePass2 now uses the .keyx extension for generated
key files, which was added to KeePassXC's key generation file chooser
filter. We continue to generate hashed binary key files by default,
but the user can explicitly save the file with the new .keyx
extension to generate an XML v2 key file (currently undocumented).
When opening a database, the key file type is still determined
by content negotation, so the file extension has no impact here.
As an additional change, the legacy key file warnings have been
improved slightly to be less confusing and more helpful.
