Display notification filter/clear actions as menu items (#3877)

Previously the notification filter and clear actions were shown as
buttons in the UI, with a preference that determined whether they were
displayed.

Remove this preference, and display them as menu items.

- "Filter notifications" is shown as an icon, if possible
- "Clear notifications" is only ever shown as a menu item, to reduce the
chance the user inadvertently selects it

To ensure that the options menu appears correctly, remove the code that
creates a "fake" action bar, and adjust the layouts so that there are
three toolbars;

- mainToolbar -- displays the icons, and the current "location" (Home,
Notifications, etc)
- topNav -- displays the row of tabs at the top
- bottomNav -- displays the row of tabs at the bottom

Only one of them is set as the support action bar (depending on the
user's preferences). This provides the "show a logo" and "show the
options menu" functionality as standard, without needing to re-implement
as the previous code did.

app/src/main/res/values-ar/strings.xml

@@ -450,7 +450,6 @@
     <string name="report_remote_instance">أعد تحويله إلى %s</string>
     <string name="failed_report">فشل الابلاغ</string>
     <string name="failed_fetch_posts">فشلت عملية جلب المنشورات</string>
-    <string name="pref_title_show_notifications_filter">اعرض مصفاة الإشعارات</string>
     <string name="filter_dialog_whole_word">الكلمة كاملة</string>
     <string name="description_poll">استطلاع رأي بالخيارات: %1$s, %2$s, %3$s, %4$s; %5$s</string>
     <string name="mute_domain_warning">هل أنت متأكد من أنك تريد حجب كافة %s؟ سوف لن يكون باستطاعتك رؤية أي محتوى قادم من هذا النطاق بعد الآن ، لا في الخيوط الزمنية العامة ولا في إخطاراتك. سيتم إزالة متابِعيك الذين هم على هذا النطاق.</string>