StackScreenNavigator: some experimentation with a stack-based screen navigator
removed IScreenNavigator interface and typed the owner property of IScreen as Object to avoid the need to cast
no need to check if the properties property of ScreenNavigatorItem is null before setting a property
StackScreenNavigatorItem: fixed naming to include 'Push' before 'Event'
TransitionsExplorer: menu now uses StackScreenNavigator
Merge branch 'master' into navigators
ScreenNavigator, StackScreenNavigator: moved addScreen(), removeScreen(), and getScreen() so that they can be typed properly to ScreenNavigtatorItem and StackScreenNavigatorItem
StackScreenNavigator: cleaned up docs and API names
ScreenNavigator: converted transition variable to a getter and setter
ScreenNavigator: converted transition variable to a getter and setter
StackScreenNavigator: added rootScreen property. changed pushTransition, popTransition, and popToRootTransition variables to getters and setters
ComponentsExplorer: converted to StackScreenNavigator
LayoutExplorer: updated to use StackScreenNavigator
cleaned up docs
LayoutExplorer: updated to use StackScreenNavigator
TrainTimes: updated to use StackScreenNavigator
TransitionsExplorer: shows transition on triggered event. fixed restoration of selected index on pop
Examples: some cleanup of adding screens to StackScreenNavigator (closes #1018)