Add title and tooltip to diff preferences dropdown

Changelog: added

app/assets/javascripts/diffs/components/settings_dropdown.vue

 <script>
-import { GlButtonGroup, GlButton, GlDropdown, GlFormCheckbox } from '@gitlab/ui';
+import {
+  GlButtonGroup,
+  GlButton,
+  GlDropdown,
+  GlFormCheckbox,
+  GlTooltipDirective,
+} from '@gitlab/ui';
 import { mapActions, mapGetters, mapState } from 'vuex';
 import { SETTINGS_DROPDOWN } from '../i18n';
 
 export default {
   i18n: SETTINGS_DROPDOWN,
+  directives: {
+    GlTooltip: GlTooltipDirective,
+  },
   components: {
     GlButtonGroup,
     GlButton,
@@ -35,9 +44,13 @@ export default {
 
 <template>
   <gl-dropdown
+    v-gl-tooltip
     icon="settings"
-    :text="__('Diff view settings')"
+    :title="$options.i18n.preferences"
+    :text="$options.i18n.preferences"
     :text-sr-only="true"
+    :aria-label="$options.i18n.preferences"
+    :header-text="$options.i18n.preferences"
     toggle-class="js-show-diff-settings"
     right
   >

app/assets/javascripts/diffs/i18n.js

@@ -23,4 +23,5 @@ export const DIFF_FILE = {
 export const SETTINGS_DROPDOWN = {
   whitespace: __('Show whitespace changes'),
   fileByFile: __('Show one file at a time'),
+  preferences: __('Preferences'),
 };

locale/gitlab.pot

@@ -11459,9 +11459,6 @@ msgstr ""
 msgid "Diff limits"
 msgstr ""
 
-msgid "Diff view settings"
-msgstr ""
-
 msgid "Difference between start date and now"
 msgstr ""