feedbackd: Support visual feedback via LEDs

Currently all LED feedbacks are infinite we can improve that
later on by using the 'repeat' attribute of the LED pattern
trigger.

README.md

@@ -80,6 +80,7 @@ available feedback types are:
 - Sound (an audible sound from the sound naming spec)
 - VibraRumble: haptic motor rumbling
 - VibraPeriodic: periodic feedback from the haptic motor
+- Led: Feedback via blinking LEDs
 
 You can check the feedback theme and the classes (prefixed with Fbd)
 for available properties. Note that the feedback theme API (including