Revert "Implements StyledNode"

This reverts commit dfa623a.

Author: singerdmx

lib/src/models/documents/nodes/block.dart

@@ -13,7 +13,7 @@ import 'node.dart';
 /// - Text Alignment
 /// - Text Direction
 /// - Code Block
-class Block extends Container<Line?> implements StyledNode {
+class Block extends Container<Line?> {
   /// Creates new unmounted [Block].
   @override
   Node newInstance() => Block();

lib/src/models/documents/nodes/leaf.dart

@@ -7,7 +7,7 @@ import 'line.dart';
 import 'node.dart';
 
 /// A leaf in Quill document tree.
-abstract class Leaf extends Node implements StyledNode {
+abstract class Leaf extends Node {
   /// Creates a new [Leaf] with specified [data].
   factory Leaf(Object data) {
     if (data is Embeddable) {

lib/src/models/documents/nodes/line.dart

@@ -17,7 +17,7 @@ import 'node.dart';
 ///
 /// When a line contains an embed, it fully occupies the line, no other embeds
 /// or text nodes are allowed.
-class Line extends Container<Leaf?> implements StyledNode {
+class Line extends Container<Leaf?> {
   @override
   Leaf get defaultChild => Text();
 

lib/src/models/documents/nodes/node.dart

@@ -127,6 +127,33 @@ abstract class StyledNode implements Node {
   Style get style;
 }
 
+/// Mixin used by nodes that wish to implement [StyledNode] interface.
+abstract class StyledNodeMixin implements StyledNode {
+  @override
+  Style get style => _style;
+  @override
+  Style _style = Style();
+
+  /// Applies style [attribute] to this node.
+  @override
+  void applyAttribute(Attribute attribute) {
+    _style = _style.merge(attribute);
+  }
+
+  /// Applies new style [value] to this node. Provided [value] is merged
+  /// into current style.
+  @override
+  void applyStyle(Style value) {
+    _style = _style.mergeAll(value);
+  }
+
+  /// Clears style of this node.
+  @override
+  void clearStyle() {
+    _style = Style();
+  }
+}
+
 /// Root node of document tree.
 class Root extends Container<Container<Node?>> {
   @override