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
Add setting for selecting automatic lpm ui triggering modes

Previously both "from-pocket" and "hover-over" proximity triggering
was enabled implicitly when use_low_power_mode setting was enabled.
The proximity sensor interaction needed for "hover-over" triggering
can be confusing and disabling it was not possible.

Now both triggering types can be disabled/enabled individually via
mcetool - for example:
* only "from-pocket"
  mcetool --set-lpmui-triggering=from-pocket
* both "from-pocket" and "hover-over"
  mcetool --set-lpmui-triggering=from-pocket,hover-over
* neither
  mcetool --set-lpmui-triggering=disabled

Default is to enable "from-pocket" only.

[mce] Add setting for selecting automatic lpm ui triggering modes. Fixes JB#20959
Fix indentation
Do not allow transient/invalid display state targets

Under certain circumstances mce will not honor display state requests
that are made. Choosing altogether different target state causes no
harm, but an attempt to keep existing state while in transitional
state causes illogical target state to be assigned and logic failures
in the underlying state machine.

Now requests that invalid from the state machine point of view are
ignored, which effectively keeps the display state unchanged.

[mce] Do not allow transient/invalid display state targets. Fixes JB#21994
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
Merge pull request #205 from spiiroin/lpm_triggering_setting

Add setting for selecting automatic lpm ui triggering modes
Merge pull request #206 from spiiroin/display_request_filtering

Do not allow transient/invalid display state targets
Bump mce version to 1.33.5