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 if enable-compat-network-schema is set or gnome-keyring has not been detected. If gnome-keyring and libsecret are both available, it will use libsecret too but with the SECRET_SCHEMA_COMPAT_NETWORK schema.