move methods on List

[robiness]

Working with reorderable lists like reorderables or ReorderableListView there often is a callback like
void onReorder(int oldIndex, int newIndex)

Would it make sense to you to add something like it's done here?

Especially the move and moveAt methods would be nice little helpers in that scenario.

[passsy]

Really like the move methods! Let the PRs fly in :)