state: adds offset access and manipulation

[wookietreiber]

Description

Adds offset access and manipulation. Fixes #495.

Testing guidelines

I have a few structs that keep the table state. I use this trait ScrollableTable for each of these to attach actions like scroll up/down, etc. I've added this fn recenter to test offset manipulation. ListState should be pretty much analogously.

Note that I haven't added tests per se, because I think that these kinds of accessor/mutator functions are quite obvious.

use tui::layout::Rect;
use tui::widgets::TableState;

pub trait ScrollableTable {
    fn size(&self) -> Rect;
    fn state(&self) -> &TableState;
    fn state_mut(&mut self) -> &mut TableState;

    fn recenter(&mut self) {
        if let Some(selected) = self.state().selected() {
            let height = self.size().height as usize;

            let offset = self.state_mut().offset_mut();
            *offset = selected.saturating_sub(height / 2);
        }
    }
}

Checklist

• I have read the contributing guidelines.
• I have added relevant tests.
• I have documented all new additions.

[wookietreiber]

cargo make does not seem to be compatible with rustc 1.56.1:

error: failed to compile `cargo-make v0.35.8`, intermediate artifacts can be found at `/tmp/cargo-install52QPLl`

Caused by:
  package `os_str_bytes v6.2.0` cannot be built because it requires rustc 1.57.0 or newer, while the currently active rustc version is 1.56.1
Error: Process completed with exit code 101.