Persist user customization of the app layout

[enlight]

• DockPanel has saveLayout() and restoreLayout(), but the state returned by saveLayout() is not serialized, so a custom serialization/deserialization mechanism has to be implemented.
• SplitPanel doesn't have saveLayout() or restoreLayout(), but relativeSizes() and setRelativeSizes() should provide sufficient information to save/restore the state, though just like DockPanel a custom serialization/deserialization mechanism will be needed.
• Custom Phosphor widgets that wrap React components will also require some work.