IBX-10391:[UDW] Set white background for tree view (#1658)

Author: OstafinL

src/bundle/Resources/public/scss/ui/modules/universal-discovery/_tree.scss

@@ -21,6 +21,7 @@
 
     .m-tree {
         border-radius: $ibexa-border-radius;
+        background: $ibexa-color-white;
     }
 
     .c-list-item__prefix-actions-item {

src/bundle/ui-dev/src/modules/content-tree/components/content-tree/content.tree.js

@@ -244,7 +244,7 @@ export default class ContentTree extends Component {
                     {this.renderHeader()}
                     {this.renderList()}
                     {this.renderLoadingSpinner()}
-                    <div className="m-tree__resize-handler" onMouseDown={this.addWidthChangeListener} />
+                    {resizable && <div className="m-tree__resize-handler" onMouseDown={this.addWidthChangeListener} />}
                 </div>
                 <div ref={this._refPopupContainer} />
             </>