Rename Panel methods (#8192)

The three methods for showing a `Panel` are now:

* `panel.show`: always show the panel.
* `panel.show_collapsible`: show or hide the panel, with a slide
animation in between.
* `Panel::show_switched`: animate between two different panels: a
thin/collapsed one and a thick/expanded one.

examples/user_attention/src/main.rs

@@ -76,7 +76,7 @@ impl eframe::App for Application {
             ));
         }
 
-        CentralPanel::default().show_inside(ui, |ui| {
+        CentralPanel::default().show(ui, |ui| {
             ui.vertical(|ui| {
                 ui.horizontal(|ui| {
                     ui.label("Attention type:");