liblights: Make use of the slow blink mode of the button backlight

Activation of the slow blink mode is synchronized with the normal
notification LED if the button backlight is off.

Change-Id: I83a603a059e1ef825413f70db2c2dfdc9c2d34bd
EndeavoruParts: add option for button light notification.

Patch Set 2: Add and update German translation

Change-Id: Iff619609daa1cf2621dadab2dfb91b64f429c114
liblights: Fix dual blink mode and add amber long blink

Change-Id: Id229bc496a5702527a7095a8b48a9cd3a0e28ab7