Remove SelectableLabel (#7277)

* part of https://github.com/emilk/egui/issues/7264
* removes SelectableLabel (Use `Button::selectable` instead)
* updates `Ui::selectable_value/label` with IntoAtoms support

Had to make some changes to `Button` since the SelecatbleLabel had no
frame unless selected.

crates/egui/src/widgets/mod.rs

@@ -22,6 +22,8 @@ mod slider;
 mod spinner;
 pub mod text_edit;
 
+#[expect(deprecated)]
+pub use self::selected_label::SelectableLabel;
 pub use self::{
     button::Button,
     checkbox::Checkbox,
@@ -35,7 +37,6 @@ pub use self::{
     label::Label,
     progress_bar::ProgressBar,
     radio_button::RadioButton,
-    selected_label::SelectableLabel,
     separator::Separator,
     slider::{Slider, SliderClamping, SliderOrientation},
     spinner::Spinner,