clean up a comment
add initial value to prevent a few unnecessary layout passes
don't need childrenAdded call for containers that are IMXMLDocuments
beginnings of major overhaul to use org.apache.flex.events.MouseEvent instead of flash.events.MouseEvent.  The 'killer' needed to be moved to the capture phase on the stage.  Some changes related to layout optimizations such as supressing some change events is included in this commit
use org.apache.flex.events.MouseEvent instead of flash.display.MouseEvent, but make org.apache.flex.events.MouseEvent extend from org.apache.flex.events.Event because some existing code has the type of the event as just Event instead of MouseEvent.  Just using Event resolves now to org.apache.flex.events.Event.  This is needed so the cross-compile isn't referencing flash.events.* classes, but means we have to copy properties from the original flash MouseEvent.
make layout a bit smarter about how often it runs.  If a component as explicitWidth/Height or percentWidth/Height, it now waits until it is sized before doing the first layout pass.  But if the component needs to size to content, it does the first layout when children are added.