Add size_hint method to Component trait

This can be used to aid the sizing strategy of
some containers directly in the component definition,
instead of stating the sizes explicitly in `view`
logic.

examples/component/src/main.rs

@@ -48,7 +48,7 @@ impl Sandbox for Component {
 mod numeric_input {
     use iced::alignment::{self, Alignment};
     use iced::widget::{button, component, row, text, text_input, Component};
-    use iced::{Element, Length};
+    use iced::{Element, Length, Size};
 
     pub struct NumericInput<Message> {
         value: Option<u32>,
@@ -143,6 +143,13 @@ mod numeric_input {
             .spacing(10)
             .into()
         }
+
+        fn size_hint(&self) -> Size<Length> {
+            Size {
+                width: Length::Fill,
+                height: Length::Shrink,
+            }
+        }
     }
 
     impl<'a, Message> From<NumericInput<Message>> for Element<'a, Message>