Use libsecret if available

This patch provides libsecret [1] support to gnucash since gnome-keyring has
been deprecated. It will check for an installed libsecret version and will use
it in favor of gnome-keyring.

Since it is not recommended to use SECRET_SCHEMA_COMPAT_NETWORK for new uses, it
uses its own defined scheme to store the credentials. Therefore passwords that
have been stored in the network schema are migrated to the new schema by trying
to access them with the old schema and storing them again in the new one.