Refactor/simplify end-session dialog handling to use a native-Cinnamon dialog if available

[mtwebster]

• Existing dialog remains as a fallback (for cinnamon fallback mode) or can be forced.
• Simplified dialog communications, use session bus instead of a custom address.
• Filter inhibitor list against logout action. If Logout, Shutdown or Reboot is requested, only list Logout inhibitors - ignore the idle inhibitor from a multimedia app.