Animation duration fix

[WieFel]

Replaced calls of controller.fling() with controller.animateTo() or controller.animateBack().
This way, animation duration + curve is now taken into account by backdrop.

The animation default curve is now set to easeInOut and the duration to 200ms (looks quite the same as before).

Fling uses its own constants for it (see https://stackoverflow.com/questions/45683716/how-to-slow-down-a-fling-animation-in-flutter), therefore the duration property wasn't working before.

Closes #4