Draft first-class Theme support

RFC: https://github.com/iced-rs/rfcs/pull/6

pure/src/widget/tooltip.rs

@@ -157,6 +157,7 @@ where
         &self,
         tree: &Tree,
         renderer: &mut Renderer,
+        theme: &Renderer::Theme,
         inherited_style: &renderer::Style,
         layout: Layout<'_>,
         cursor_position: Point,
@@ -165,6 +166,7 @@ where
         self.content.as_widget().draw(
             &tree.children[0],
             renderer,
+            theme,
             inherited_style,
             layout,
             cursor_position,
@@ -191,6 +193,7 @@ where
                     tooltip,
                     &Tree::empty(),
                     renderer,
+                    theme,
                     defaults,
                     layout,
                     cursor_position,