Make weight configurable when placed in sidebar

The following translated strings have been changed/added:

• Settings summary: "Placed in sidebar" --> "Placed in sidebar (weight: @weight)"
• Settings form sidebar_weight: "Defines the position in the sidebar where the field will be placed. Default is at the end.
  To show above the path alias use a value < @pathalias_weight and to show above author information use a value < @author_weight."

I leave it to the project maintainers to reword them. The translations need to be adapted accordingly.

Closes #3452518

src/Plugin/Field/FieldWidget/TaxonomyWidgetPlus.php

@@ -21,6 +21,11 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
  */
 class TaxonomyWidgetPlus extends WidgetBase {
 
+  /**
+   * The default weight of the widget when placed in the sidebar.
+   */
+  const SIDEBAR_DEFAULT_WEIGHT = 100;
+
   /**
    * The entity type manager.
    *
@@ -64,6 +69,7 @@ class TaxonomyWidgetPlus extends WidgetBase {
     return [
       'description_on_hover' => TRUE,
       'sidebar' => TRUE,
+      'sidebar_weight' => self::SIDEBAR_DEFAULT_WEIGHT,
       'use_scrollbar' => TRUE,
       'start_collapsed' => FALSE,
       'hide_parent_field' => FALSE,
@@ -98,6 +104,23 @@ class TaxonomyWidgetPlus extends WidgetBase {
       '#default_value' => $this->getSetting('sidebar'),
     ];
 
+    $field_name = $this->fieldDefinition->getName();
+    $form['sidebar_weight'] = [
+      '#type' => 'weight',
+      '#title' => $this->t('Sidebar weight'),
+      '#description' => $this->t('Defines the position in the sidebar where the field will be placed. Default is at the end.<br>To show above the path alias use a value < @pathalias_weight and to show above author information use a value < @author_weight.',
+      [
+        '@pathalias_weight' => 30,
+        '@author_weight' => 90,
+      ]),
+      '#default_value' => $this->getSetting('sidebar_weight') ?? self::SIDEBAR_DEFAULT_WEIGHT,
+      '#states' => [
+        'visible' => [
+          ":input[name='fields[$field_name][settings_edit_form][settings][sidebar]']" => ['checked' => TRUE],
+        ],
+      ],
+    ];
+
     $form['use_scrollbar'] = [
       '#type' => 'checkbox',
       '#title' => $this->t('Use scrollbar'),
@@ -133,7 +156,8 @@ class TaxonomyWidgetPlus extends WidgetBase {
     }
 
     if ($this->getSetting('sidebar')) {
-      $summary[] = $this->t('Placed in sidebar');
+      $weight = $this->getSetting('sidebar_weight') ?? self::SIDEBAR_DEFAULT_WEIGHT;
+      $summary[] = $this->t('Placed in sidebar (weight: @weight)', ['@weight' => $weight]);
     }
 
     if ($this->getSetting('use_scrollbar')) {
@@ -188,7 +212,7 @@ class TaxonomyWidgetPlus extends WidgetBase {
     $sidebar = $this->getSetting('sidebar');
     if ($sidebar) {
       $element['#group'] = 'advanced';
-      $element['#weight'] = 100;
+      $element['#weight'] = $this->getSetting('sidebar_weight') ?? self::SIDEBAR_DEFAULT_WEIGHT;
     }
 
     $element['#element_validate'] = [