Fix issue of add/remove a frame at a scene before all key frames of a layer. When you added or removed a frame at a scene before all key frames of a layer, frameline was not updated correctly. It seems nothing happened, but domview is updated. This changeset fix this issue by correcting logic for boundary case.

content	MadBuilder	content/
overlay	chrome://browser/content/browser.xul	chrome://MadBuilder/content/MadBuilderOverlay.xul