* 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.
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