MD: fix page controller updates

[jpnurmi]

MasterDetailPage can be rebuilt with either a) a different controller, or b) a different length and initialIndex. Make sure didUpdateWidget checks for both cases.

Ref: ubuntu-flutter-community/settings#421

[Feichtmeier]

Thank you! Maybe you were right after all and we should just add a required controller some day 🤠

[jpnurmi]

It was the opposite, actually. 🙈 A controller was required in the beginning because it made our internal implementation simple. However, it was inconvenient to use because often you don't really need one. Therefore I proposed making it easier for the user by allowing them to pass an initial index and length just like they did before, at the cost of making our internal implementation more complex...