This is a ground work for adding a local storage backend. As
SecretItem is derived from GDBusProxy, it cannot be simply exposed to
the application through the secret_password_search() if the item is
not backed by the DBus API. This adds an abstract interface
representing a read-only view of a secret item for that purpose.
Added macros:
* SECRET_VERSION_MAJOR
* SECRET_VERSION_MINOR
* SECRET_VERSION_MICRO
* SECRET_CHECK_VERSION
These macros are widely defined in GLib based library. For example,
GLib, GTK, poppler GLib and so on define them.
These macros are useful to detect libsecret version on build type and
from GObject Introspection based bindings.
* C callers need to use libsecret-0 pkg-config file for stable and
libsecret-unstable for unstable stuff.
* Vala callers need to '--pkg libsecret-unstable' for unstable
* GObject Introspection callers need to use the SecretUnstable package
* Death by a thousand paper cuts from gir and vapi not liking
the fact that the secret.h file was not usable uninstalled
and installed in the same way.
