Persist sketch parameters across app relaunch

[abey79]

BREAKING: this change adds the requirement for all sketches to implement serde::{Serialize, Deserialize}. To minimise the related boilerplate, new #[sketch_app] and #[sketch_widget] attributes are introduced—see below.

This PR adds persistance of the sketch parameters across relaunches of the app. Resetting the parameters to their default is now done using the reset button (see #91).

Serialising the sketch requires it to implement the serde::{Serialize, Deserialize} traits. To minimise the related boilerplate (including adding serde as dependency), a new #[sketch_app] attribute proc macro is introduced.

The following code:

#[sketch_app]
struct MySketch {
    // ...
}

is now equivalent to:

#[derive(Sketch, serde::Serialize, serde::Deserialize)]
#[serde(crate = ::whiskers::prelude::serde)]
struct MySketch {
    // ...
}

The #[serde(...)] attribute is needed to point the derive macro at the whiskers-exported serde crate.

Likewise, a new #[sketch_widget] attribute macro is introduced to derive the Widget trait along with serde's.

This PR is also a pre-requisite for saved configurations.