External applications now accept containers which function as the
frontend where the ExtApplication functions as the backend. Containers
execute within their own engine and have their own context and are
destroyed when released.
- solves the problem of non-standard installs
- add AssetPath to config as string var
- introduced UiLib constructor which falls back to defaultAssetPath (earlier behaviour) if no assetPath is set
- defaultAssetPath now internal concern of UiLib
- gui.Start(assetPath) argument passed from ethereal main() as set Init() in config.go
- informative log message if wallet.qml fails to open
