Rename classes, more button options via StyleProvider (#8464)

Moves global classnames to e.g. `egui::class::ROOT`.

Widget specific classes live on the widgets struct. 

Adds some more classes for our buttons styling options (`CLASS_SMALL`
etc), so that custom `StyleProvider`s can choose to implement them
differently or ignore them altogether.


Also removes some styleproviders that were partly or completely unused
(e.g. basestyle / labelstyle). We can add them back once we actually
implement styling for those.

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/egui_tests/tests/test_atoms.rs

@@ -133,7 +133,7 @@ fn test_atom_layout_nesting_and_direction() {
 
         let button_frame = ui
             .get_widget_style::<ButtonStyle>(&StyleArgs {
-                classes: &egui::widget_style::Classes::default(),
+                classes: &egui::class::Classes::default(),
                 state: egui::widget_style::WidgetState::Inactive,
                 ctx: ui,
                 stack: ui.stack(),