Expose function helpers to build widgets in pure::widget

`button("Hello")` is easier to write and read than
`Button::new("Hello")`.

examples/pure/counter/src/main.rs

@@ -1,4 +1,5 @@
-use iced::pure::{Button, Column, Element, Sandbox, Text};
+use iced::pure::widget::{button, column, text};
+use iced::pure::{Element, Sandbox};
 use iced::{Alignment, Settings};
 
 pub fn main() -> iced::Result {
@@ -38,12 +39,12 @@ impl Sandbox for Counter {
     }
 
     fn view(&self) -> Element<Message> {
-        Column::new()
+        column()
             .padding(20)
             .align_items(Alignment::Center)
-            .push(Button::new("Increment").on_press(Message::IncrementPressed))
+            .push(button("Increment").on_press(Message::IncrementPressed))
-            .push(Text::new(self.value.to_string()).size(50))
+            .push(text(self.value).size(50))
-            .push(Button::new("Decrement").on_press(Message::DecrementPressed))
+            .push(button("Decrement").on_press(Message::DecrementPressed))
             .into()
     }
 }

pure/src/widget.rs

@@ -71,3 +71,20 @@ pub trait Widget<Message, Renderer> {
         event::Status::Ignored
     }
 }
+
+pub fn column<Message, Renderer>() -> Column<Message, Renderer> {
+    Column::new()
+}
+
+pub fn button<Message, Renderer>(
+    content: impl Into<Element<Message, Renderer>>,
+) -> Button<Message, Renderer> {
+    Button::new(content)
+}
+
+pub fn text<Renderer>(text: impl ToString) -> Text<Renderer>
+where
+    Renderer: iced_native::text::Renderer,
+{
+    Text::new(text)
+}

pure/src/widget/text.rs

@@ -24,9 +24,9 @@ where
 
 impl<Renderer: text::Renderer> Text<Renderer> {
     /// Create a new fragment of [`Text`] with the given contents.
-    pub fn new<T: Into<String>>(label: T) -> Self {
+    pub fn new<T: ToString>(label: T) -> Self {
         Text {
-            content: label.into(),
+            content: label.to_string(),
             size: None,
             color: None,
             font: Default::default(),