Allow multiple atoms in Button::shortcut_text and right_text (#7696)

Useful when intermixing text and icons (e.g. for modifiers)

tests/egui_tests/tests/snapshots/button_shortcut.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:5befd84158b582c79a968f36e43c7017187b364824eb4470b048d133e62f9360
+size 1600

tests/egui_tests/tests/test_atoms.rs

@@ -108,3 +108,14 @@ fn test_intrinsic_size() {
         }
     }
 }
+
+#[test]
+fn test_button_shortcut_text() {
+    let mut harness = HarnessBuilder::default().build_ui(|ui| {
+        ui.add(egui::Button::new("Click me").shortcut_text(("1", "2", "3")));
+    });
+    harness.run();
+    harness.fit_contents();
+
+    harness.snapshot("button_shortcut");
+}