[REF] *: BS4, adapt dropdowns and carets

- The dropdown structure was simplified, allowing to get rid of the
  3-levels structure induced by <ul/> elements and dropdowns can now
  contain anything. The class 'dropdown-item' is now mandatory for
  each dropdown clickable element. The class 'dropdown-item-text' can
  be used to add same padding and style but without making the element
  have a clickable look.

- Dividers now use the class 'dropdown-divider'

- The way dropdowns are opened and hidden also changed (before the
  'open' class was added on the `.dropdown-menu` parent, now the
  'show' class is added on both the `.dropdown-menu` parent and the
  `.dropdown-menu` itself).

- JS-wise, no click event handlers can be put on `.dropdown-toggle`
  elements anymore (instead, use handlers for dropdown events).

- Carets are automatically put on `.dropdown-toggle` elements, so this
  commit replaces the `.caret` elements with this. This feature was
  possible to disable but would prevent us from adding a caret with
  scss. Also, this simplifies the DOM. The 'o-no-caret' class was also
  introduced to allow using the 'dropdown-toggle' class on non-caret
  elements.

- Also adapt the scss to use $caret-width instead of $caret-width-base

addons/web/static/src/js/fields/basic_fields.js

@@ -1612,10 +1612,7 @@ var AttachmentImage = AbstractField.extend({
 var StateSelectionWidget = AbstractField.extend({
     template: 'FormSelection',
     events: {
-        'click a': function (e) {
-            e.preventDefault();
-        },
-        'click li': '_setSelection'
+        'click .dropdown-item': '_setSelection',
     },
     supportedFieldTypes: ['selection'],
 
@@ -1696,13 +1693,12 @@ var StateSelectionWidget = AbstractField.extend({
      * @param {MouseEvent} ev
      */
     _setSelection: function (ev) {
-        var li = $(ev.target).closest('li');
-        if (li.length) {
-            var value = String(li.data('value'));
-            this._setValue(value);
-            if (this.mode === 'edit') {
-                this._render();
-            }
+        ev.preventDefault();
+        var $item = $(ev.currentTarget);
+        var value = String($item.data('value'));
+        this._setValue(value);
+        if (this.mode === 'edit') {
+            this._render();
         }
     },
 });