Use Self everywhere (#3787)

This turns on the clippy lint
[`clippy::use_self`](https://rust-lang.github.io/rust-clippy/v0.0.212/index.html#use_self)
and fixes it everywhere.

crates/egui_demo_app/src/apps/image_viewer.rs

@@ -27,9 +27,9 @@ enum ChosenFit {
 impl ChosenFit {
     fn as_str(&self) -> &'static str {
         match self {
-            ChosenFit::ExactSize => "exact size",
-            ChosenFit::Fraction => "fraction",
-            ChosenFit::OriginalSize => "original size",
+            Self::ExactSize => "exact size",
+            Self::Fraction => "fraction",
+            Self::OriginalSize => "original size",
         }
     }
 }

crates/egui_demo_app/src/backend_panel.rs

@@ -35,7 +35,7 @@ enum RunMode {
 ///    so there are no events to miss.
 impl Default for RunMode {
     fn default() -> Self {
-        RunMode::Reactive
+        Self::Reactive
     }
 }
 
@@ -242,7 +242,7 @@ struct EguiWindows {
 
 impl Default for EguiWindows {
     fn default() -> Self {
-        EguiWindows::none()
+        Self::none()
     }
 }
 

crates/egui_demo_app/src/wrap_app.rs

@@ -94,14 +94,14 @@ impl Anchor {
     #[cfg(target_arch = "wasm32")]
     fn all() -> Vec<Self> {
         vec![
-            Anchor::Demo,
-            Anchor::EasyMarkEditor,
+            Self::Demo,
+            Self::EasyMarkEditor,
             #[cfg(feature = "http")]
-            Anchor::Http,
-            Anchor::Clock,
+            Self::Http,
+            Self::Clock,
             #[cfg(any(feature = "glow", feature = "wgpu"))]
-            Anchor::Custom3d,
-            Anchor::Colors,
+            Self::Custom3d,
+            Self::Colors,
         ]
     }
 }