Add factory constructors to distinguish between timer based or network based instantiation

[fezu54]

The current SplashScreen constructor requires the second property although it is not needed when the SplashScreen uses the future based navigation. Hence, I've removed the @required annotation and created two factory constructors which makes the API a bit more self explaining.

• add factory constructors to either require seconds for timer based instantiation or the navigateFuture for network call based instantiation

• remove required annotation for seconds on existent constructor

• make existent constructor protected for backward compatibility and smooth migration

• fix broke pubspec.lock.yml

• exclude Idea's *.iml files from being pushed to git

[ahmedmgh67]

This looks like an awesome contribution, I'm reviewing it now to see if we'll merge it. Thanks