fix: initial screen and its model not disposing

[programadorthi]

This fix is because Navigator dispose checks the last disposed or replaced screen. But first navigator screen will be never popped or replaced because Navigator needs at least one screen to work. Pressing back button or closing applications never dispose first screen.
So here we are putting a effect to dispose based on Navigator been replaced or application been finished. And if a Navigator is totally replaced by another (yes, we can't avoid users does that) it'll will dispose all pushed screens.