Lexical: Updated dropdown handling to match tinymce behaviour

Now toolbars stay open on mouse-out, and close on other toolbar open,
outside click or an accepted action.
To support:
- Added new system to track and manage open dropdowns.
- Added way for buttons to optionally emit events upon actions.
- Added way to listen for events.
- Used the above to control when dropdowns should hide on action, since
  some dont (like overflow containers and split dropdown buttons).

resources/js/wysiwyg/ui/framework/buttons.ts

@@ -10,7 +10,12 @@ export interface EditorBasicButtonDefinition {
 }
 
 export interface EditorButtonDefinition extends EditorBasicButtonDefinition {
-    action: (context: EditorUiContext, button: EditorButton) => void;
+    /**
+     * The action to perform when the button is used.
+     * This can return false to indicate that the completion of the action should
+     * NOT be communicated to parent UI elements, which is what occurs by default.
+     */
+    action: (context: EditorUiContext, button: EditorButton) => void|false;
     isActive: (selection: BaseSelection|null, context: EditorUiContext) => boolean;
     isDisabled?: (selection: BaseSelection|null, context: EditorUiContext) => boolean;
     setup?: (context: EditorUiContext, button: EditorButton) => void;
@@ -78,7 +83,10 @@ export class EditorButton extends EditorUiElement {
     }
 
     protected onClick() {
-        this.definition.action(this.getContext(), this);
+        const result = this.definition.action(this.getContext(), this);
+        if (result !== false) {
+            this.emitEvent('button-action');
+        }
     }
 
     protected updateActiveState(selection: BaseSelection|null) {