On Windows, add option to customize window class name (#2978)

src/platform_impl/windows/mod.rs

@@ -30,6 +30,7 @@ pub struct PlatformSpecificWindowBuilderAttributes {
     pub no_redirection_bitmap: bool,
     pub drag_and_drop: bool,
     pub skip_taskbar: bool,
+    pub class_name: String,
     pub decoration_shadow: bool,
 }
 
@@ -42,6 +43,7 @@ impl Default for PlatformSpecificWindowBuilderAttributes {
             no_redirection_bitmap: false,
             drag_and_drop: true,
             skip_taskbar: false,
+            class_name: "Window Class".to_string(),
             decoration_shadow: false,
         }
     }