Config editor; revise model traits, Spinner widget, SizeMgr::dpem, ...

[dhardy]

The highlight of this PR is a new editor for event config:

The code for this in the gallery is tiny; essentially just:

let config = toolkit.event_config().clone();
let editor: SingleView<SharedRc<Config>> = SingleView::new(config);

Instead the editor implementation is an impl of kas::widgets::view::driver::Driver for driver::DefaultView, and thus easy to re-use.

Quite a few other changes happened as part of this work:

• Add kas_core::label::StrLabel: a hidden simple label widget used for macro-generated implicit widgets from string literals (rationale: this feature is now usable within kas_widgets)
• Replace Vec2::max_abs_comp with max_comp (can be used with Vec2::abs)
• Add TkAction::EMPTY
• Revise Spinner widget: bug-fix + cleaner code
• Rename trait SpinnerType → SpinnerValue and SliderType → SliderValue
• Slider and Spinner now have optional event-handling closures like Button, CheckBox etc.
• Replace usages of Rc<RefCell<Config>> with SharedRc<Config> and update derived values when the shared value is updated
• SharedRc: add borrow, try_borrow, update_mut methods (eventually these should move to the SharedData trait but that requires GATs)
• SharedRc: remove ListData and MatrixData impls
• Rename trait SingleData to SharedData (and *Mut version); use as base of ListData, MatrixData: this means we now have a useful trait as the basis of all shared data
• Add Data: SharedData as a type parameter to Driver; add Driver::on_message to handle event messages (this replaces SingleData::handle_message etc.)
• Make kas_widgets::edit a public module containing EditField and associates
• Add SizeMgr::dpem, min_scroll_size; remove pixels_from_* methods and line_height
• Revise and unify handling of scroll-wheel actions (using event config)