Improved view ergonomics

[hecrj]

This PR improves some of the ergonomics of view code:

• The Fill, FillPortion, and Shrink variants of Length are re-exported by the root crate. Effectively, this means we can now import these directly:

use iced::widget::container;
use iced::Fill;

container("Yay!").width(Fill)

• The Center, Left, Right, Top, and Bottom variants of the different alignment enums are also re-exported. Thus, we can now write:

use iced::widget::text;
use iced::{Center, Fill};

text("I am centered!").width(Fill).align_x(Center)

• align_items in Column and Row have been renamed to align_x and align_y respectively.
• horizontal_alignment and vertical_alignment in Text have been renamed to align_x and align_y.
• The padding module exposes free functions to build Padding values and the type has methods of the same name that work as setters. Thus, we can write:

use iced::widget::container;
use iced::padding;

container("I have a padding of 10 all around, except my right side!")
    .padding(padding::all(10).right(0))

• The border module has received a similar treatment to padding:

use iced::widget::container;
use iced::border;

container("My top right corner isn't rounded!")
    .style(|theme: &Theme| container::Style {
        border: border::rounded(border::radius(10).top_right(0)).color(Color::BLACK).width(2),
        ..container::Style::default()
    })

Closes #2260.

[derezzedex]

Can we do the same for ContentFit?

[hecrj]

I don't think those are that common, since they are only used for images.