Introduce subscription::Event

... and remove `PlatformSpecific` from `Event`

widget/src/canvas.rs

@@ -172,7 +172,7 @@ where
             core::Event::Keyboard(keyboard_event) => {
                 Some(Event::Keyboard(keyboard_event))
             }
-            _ => None,
+            core::Event::Window(_) => None,
         };
 
         if let Some(canvas_event) = canvas_event {

widget/src/shader.rs

@@ -110,7 +110,7 @@ where
             core::Event::Window(window::Event::RedrawRequested(instant)) => {
                 Some(Event::RedrawRequested(instant))
             }
-            _ => None,
+            core::Event::Window(_) => None,
         };
 
         if let Some(custom_shader_event) = custom_shader_event {