Re-organize custom module as pipeline module

... and move `Shader` widget to `iced_widget` crate

widget/src/shader/event.rs

@@ -0,0 +1,26 @@
+//! Handle events of a custom shader widget.
+use crate::core::keyboard;
+use crate::core::mouse;
+use crate::core::touch;
+
+use std::time::Instant;
+
+pub use crate::core::event::Status;
+
+/// A [`Shader`] event.
+///
+/// [`Shader`]: crate::widget::shader::Shader;
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Event {
+    /// A mouse event.
+    Mouse(mouse::Event),
+
+    /// A touch event.
+    Touch(touch::Event),
+
+    /// A keyboard event.
+    Keyboard(keyboard::Event),
+
+    /// A window requested a redraw.
+    RedrawRequested(Instant),
+}