Some Scrolling tweaks

[13r0ck]

While all of the changes work well, I am quite new to Rust, and I imagine that many changes will have to be made for the code quality to meet the rest of the Iced code base.

3 things in this pull request:

1. Adding scroll to pick_lists
   I think that this functionality for pick_lists should be expected. Just allows for selecting from a pick_list by scrolling over a closed pick_list. Though I am not confident with my code quality. It works reliably, but code is a mess. Please let me know how to improve it.

2. Adding on_scroll functions to text_input
   I added on_scroll_up and on_scroll_down to Text_Inputs. I am not confident on implementation nor code quality on this one. I needed it on my project specifically for Text_Inputs that the user enters integers, and then scroll for tweaks if wanted (think a timer input that allows to typing in the time as well as scrolling to the time). These functions could also be useful for other widgets (on_scroll_left could be useful for a slider for example). Though I am open to any code implementation ideas.

3. Adding convenience functions for scrollables
Simple functions just for convenience, rather than having to play with Rectangles and content, just one function call to scroll to the bottom. (I use this on a GUI that creates new elements in a list above the create button. The create button is always at the bottom of a Scrollable thus scrolling to the bottom after pressing the create button is convenient).
Implemented by #607

[yusdacra]

Have you seen #607 ?

[13r0ck]

Oh no, I haven't. Thanks for bringing it up. #607 is better than my implementation.
I'll drop the third change from my commit

[13r0ck]

I had a discussion on discord about this commit. They had suggested that the changes I made to the text_input makes more sense in a new Sequential widget. I'm open to the idea, and willing to make it, just looking for some input on what that should look like?

Also would it still be valuable to add on_scroll methods to other widgets like Sliders?

[13r0ck]

Closed by #872