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-fa/strings.xml

@@ -398,7 +398,6 @@
     <string name="failed_fetch_posts">شکست در واکشی فرسته‌ها</string>
     <string name="title_accounts">حساب‌ها</string>
     <string name="failed_search">شکست در جست‌وجو</string>
-    <string name="pref_title_show_notifications_filter">نمایش پالایهٔ آگاهی‌ها</string>
     <string name="create_poll_title">نظرسنجی</string>
     <string name="duration_5_min">۵ دقیقه</string>
     <string name="duration_30_min">۳۰ دقیقه</string>
@@ -684,4 +683,4 @@
     <string name="about_copied">اطّلاعات افزاره و نگارش رونوشت شد</string>
     <string name="list_exclusive_label">نهفتن از خط زمانی خانگی</string>
     <string name="error_media_playback">پخش شکست خورد: %s</string>
-</resources>
+</resources>