* Add all relevant message constants.
* Implement the messaging to send/retrieve key info.
* Implement _Flatten/_Unflatten for sending flat BKey objects.
* Remove application list from BKey, the key can't only differ by
allowed applications as the identifiers would still collide, so the
comparison isn't needed to uniquely identify the key. The applications
can be enumerated via the BKeyStore instead.
* Modified the API greatly to be based on BKey* instead of BPassword*.
* Added BKeyPurpose and used it instead of BKeyType. It is supposed to
indicate the purpose of a key so that an app can look up keys on a
more granular level. The BKeyType on the other hand actually
identifies the type (i.e. subclass of BKey) so an app knows how to
handle a given key or may only enumerate/use keys it is compatible
with.
* Made everything based on a raw data buffer for now, only BPasswordKey
is implemented yet which stores the (0 terminated) string into that
data buffer.
* Removed the additional data BMessage as I don't yet see where it fits
in. While I could imagine adding meta data to a key may be nice it
might be an interoperability concern when keys are shared by
different apps.
* Moved the app functions to the keystore as per the TODO, but not sure
how to actually implement them.
