Feature: Improve calculations of scrolling outside or inside

[karvulf]

Currently there are two ways of getting the current scroll value:

• inside ReorderableBuilder or outside
• if it's inside, then ScrollController is required and should be added
• if it's outside, then ScrollController shouldn't be added because it causes issues

It would be nice if there is a way either of not adding any ScrollController by finding them maybe in the context or the way of getting the scroll value is always the same, e.g. you have always to add ScrollController (outside or inside).
Like this the behavior is more consistent and would probably cause less bugs because I have to focus on only one value.

[karvulf]

Because it isn't possible to find the ScrollController, if the child has it, this issue can be closed. The rest was improved and better explained with this PR #154