Insteon: Use Relative State Change on Manual Dimming

Determine relative percentage change based on interval between start and stop manual changes

Set device's state based on relative change, no longer request state after change, cuts down on excess messages

Set linked device states based on relative change, this permits more accurate representation of dimming of a scene with varying on_levels.  For Example, imagine a Scene where Light1 is linked to Light2 such that Light2's on_level is 75%.

After the scene is turned on, Light1 is then manually dimmed by 15%, Light2 will then dim by 15% as well and its state will be updated.

The resulting states would be Light1 85% and Light2 60%.