Make mce_gconf_notifier_remove() friendlier for normal uses

No need to litter code with GPOINTER_TO_INT() and dummy parameters
just to make the function compatible with g_slist_foreach()

Add separate helper function mce_gconf_notifier_remove_cb() for the
one exceptional case that actually uses g_slist_foreach().

Also make sure mce_gconf_notifier_add() always sets the resulting
notification id before returning to the caller and allow calling
mce_gconf_notifier_remove() with zero id without ill effects.
Do not exit / disable plugins if builtin gconf keys are missing

In general all configurable items have sane built-in defaults and
thus not having configuration keys available must not result in
termination of mce process or rejecting loaded plugins.

Since unwanted exits have been removed, mce_gconf_notifier_add()
can now return actual success/failure value - except for the
gconf disabled on purpose situation.

Also normalize places where mce_gconf_notifier_add() is used so that
initial value is read after installing change notifier callback.
Suppress warnings about missing led configuration toggles

Since custom led patterns can be added via ini-files and enable/disable
configuration deals with default patterns only, mce should not complain
about missing led configuration keys by default.

[mce] Suppress warnings about missing led configuration toggles. Fixes JB#20539
Merge pull request #181 from spiiroin/led_config_toggles

Led config toggles