[major_components] back layer nuances

[micimize]

I've finally gotten the top bar behavior down by through alwaysOnTopGlobalKey, and the front layer default vertical behavior is well behaved because of InPlaceHandoffPageRoute, SlideVerticallyBetweenPages and FadeBetweenPages.

However, the backLayer can have a slurry of local, global, or shared widgets that could all have different behaviors. Maybe we can use heroes here though, idk.