top-panel: allow device suspend from power menu

Add suspend from the top-panel power menu alongside shutdown and logout. This allows far quicker access than waiting for the automatic timeout or typing the command.

As gnome-session/session-manager does not expose a suspend option via DBus, this has been implemented under the screen saver manager which already has the required code for access to logind.

TODO:

• Rebase on current main and move to PhoshSuspendManager
• Use or otherwise remove the on_suspend_finished callback.
• Implement suspend as an action.
• Test whether an interactive suspend request would be better
• Implement something like the end_session_dialog to handle inhibitors (currently this is a hard suspend)
• Actually test on a device