Add placeholder support to text_editor widget

core/src/renderer/null.rs

@@ -118,6 +118,10 @@ impl text::Editor for () {
 
     fn with_text(_text: &str) -> Self {}
 
+    fn is_empty(&self) -> bool {
+        true
+    }
+
     fn cursor(&self) -> text::editor::Cursor {
         text::editor::Cursor::Caret(Point::ORIGIN)
     }

core/src/text/editor.rs

@@ -13,6 +13,9 @@ pub trait Editor: Sized + Default {
     /// Creates a new [`Editor`] laid out with the given text.
     fn with_text(text: &str) -> Self;
 
+    /// Returns true if the [`Editor`] has no contents.
+    fn is_empty(&self) -> bool;
+
     /// Returns the current [`Cursor`] of the [`Editor`].
     fn cursor(&self) -> Cursor;