Implement remaining GameMode APIs

- [ ] Property `ClientCount`
- [ ] Method `RefreshConfig`
- [ ] Method `ListGames` - requires publishing `Game` objects with
  - [ ] Property `ProcessId`
  - [ ] Property `Executable`
  - [ ] Property `Requester`
  - [ ] Property `Timestamp`
- [ ] Signal `GameRegistered`
- [ ] Signal `GameUnregistered`