Allow going to lpm mode when display off request is made over D-Bus

If set to "use-lpm", any display off requests made over D-Bus are
handled as if MCE_DISPLAY_LPM_ON were requested.

Default to "disabled", i.e. display off requests are honored.

[mce] Allow going to lpm mode when display off request is made over D-Bus. Fixes JB#21918
Remove config notifiers when display plugin is unloaded

After the module is unloaded, unexpected things can happen if
stale callback function pointers are left active.

Now all config change notifiers should be removed on plugin unload.
Merge pull request #204 from spiiroin/display_off_override

Allow going to lpm mode when display off request is made over D-Bus