Improve splash dialogs

[Bungeefan]

The current version of our splash dialogs was an improvised solution because of a missing Navigator.
As we now have a navigator since 6f8d628, we could implement proper Dialogs again including Animations.

[rjahn]

What's the problem without navigator? Some details please.

[Bungeefan]

A simple Stack doesn't AFAIK show/allow any animations when inserting a dialog, as these are, at least partly, handled by the respective underlying route that is missing in this case, due to using a stack instead of the navigator.
Using the now existing navigator to display dialogs, would remove this workaround and allow for future animations.

However, as these dialogs are very state driven, it would require some tinkering to introduce reliable navigator dialogs, if possible/useful at all.