leds: Separate single color from multicolor leds

Turn the FbdDevLed into a class and allow FbdDevMulticolor to derive form it. This avoids special cases for the later.

The order of things is as follows:

• move FbdDevLed to a separate file
• encapsulte FbdDevLed attributes until we can make the struct opaque
• turn FbdDevLed into GObject
• Make FbdDevLed a GInitable so initialization can fail without having all in fbd_dev_led_new () (thus making things harder for derived classes)
• Add a minimal set of class methods with a default implementation (that still doesn't change the way things work)
• Extract multicolor specific bits into a derived class thus making the default implementations simpler

I haven't yet tested the multicolor bits on a multicolor led system but single color still works as expected. I'll give this a second pass to cleanup some details but wanted to get this out so !100 (merged) can move forward.