Deprecate ui.set_enabled and set_visbile (#4614)

These were confusing, because `set_enabled(true)` and
`set_visible(true)` did nothing.

Instead use one of:
* `ui.add_enabled`, `ui.add_enabled_ui` or `ui.disable()`
* `ui.add_visible`, `ui.add_visible_ui` or `ui.set_invisible()`

* Closes https://github.com/emilk/egui/issues/4327

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -62,7 +62,9 @@ impl super::Demo for WidgetGallery {
 impl super::View for WidgetGallery {
     fn ui(&mut self, ui: &mut egui::Ui) {
         ui.add_enabled_ui(self.enabled, |ui| {
-            ui.set_visible(self.visible);
+            if !self.visible {
+                ui.set_invisible();
+            }
             ui.multiply_opacity(self.opacity);
 
             egui::Grid::new("my_grid")

crates/egui_demo_lib/src/demo/window_options.rs

@@ -115,7 +115,9 @@ impl super::View for WindowOptions {
             ui.group(|ui| {
                 ui.vertical(|ui| {
                     ui.checkbox(anchored, "anchored");
-                    ui.set_enabled(*anchored);
+                    if !*anchored {
+                        ui.disable();
+                    }
                     ui.horizontal(|ui| {
                         ui.label("x:");
                         ui.selectable_value(&mut anchor[0], egui::Align::LEFT, "Left");