Towards an unified and clean `Widget` methods?

Nobody has claimed this yet.

The `Widget` trait have many complex methods, and each has many different parameters:

```rust
fn event(&mut self, ctx: &mut EventCtx, event: &Event, data: &mut T, env: &Env);
fn lifecycle(&mut self, ctx: &mut LifeCycleCtx, event: &LifeCycle, data: &T, env: &Env);
fn update(&mut self, ctx: &mut UpdateCtx, old_data: &T, data: &T, env: &Env);
fn layout(&mut self, ctx: &mut LayoutCtx, bc: &BoxConstraints, data: &T, env: &Env) -> Size;
fn paint(&mut self, ctx: &mut PaintCtx, data: &T, env: &Env);
```

Maybe some of its parameters (such as `event` `data` `env` `bc` `old_data`) can be merged into Context types? Then we will get an more unified and clean methods:

```rust
fn event(&mut self, ctx: &mut EventCtx);
fn lifecycle(&mut self, ctx: &mut LifeCycleCtx);
fn update(&mut self, ctx: &mut UpdateCtx);
fn layout(&mut self, ctx: &mut LayoutCtx) -> Size;
fn paint(&mut self, ctx: &mut PaintCtx);
```

Start with the Widget trait methods shown in the issue: event, lifecycle, update, layout, and paint. Review how their context and non-context parameters are used across the toolkit; done would require an agreed and tested API redesign, but the issue names no files or tests.

Written by the indexing model from the issue text.