secret-backend: Add local-storage backend
This series allows applications to store passwords in a local file, rather than through the D-Bus interface. This is mainly for https://github.com/flatpak/xdg-desktop-portal/issues/14.
When linked with a flatpak application, libsecret by default tries to use the local file. The master password to encrypt the file is retrieved through a portal interface.
Edited by Daiki Ueno