Monitor and respect logind idle inhibitor

[ids1024]

It seems there are four slightly different mechanisms for inhibiting idle:

• idle-inhibit-unstable-v1 Wayland protocol
  • Handled by the compositor, impacting the behavior of the idle notifier
• org.freedesktop.ScreenSaver dbus protocol
  • Now implemented by cosmic-idle
• org.freedesktop.portal.Inhibit portal
  • xdg-desktop-portal-gtk implements this using org.freedesktop.portal.Inhibit
    • https://github.com/flatpak/xdg-desktop-portal-gtk/blob/main/src/inhibit.c
    • The portal supports inhibitors other than idle, but the xdg-desktop-portal-gtk implementation doesn't; so we'll need our own implementation if we want those
• Logind inhibitor locks

Not sure exactly what clients use which. Swayidle added code to respect logind idle inhibitor locks in swaywm/swayidle#48.