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-hi/strings.xml

@@ -256,7 +256,6 @@
     <string name="no_drafts">आपके पास कोई ड्राफ्ट नहीं है।</string>
     <string name="post_lookup_error_format">%s पोस्ट खोजने में त्रुटि</string>
     <string name="pref_title_enable_swipe_for_tabs">टैब के बीच स्विच करने के लिए स्वाइप जेस्चर को सक्षम करें</string>
-    <string name="pref_title_show_notifications_filter">सूचना फ़िल्टर दिखाएं</string>
     <string name="failed_search">खोज करने में विफल</string>
     <string name="report_description_remote_instance">खाता किसी अन्य सर्वर से है। रिपोर्ट की अनाम प्रति वहां भी भेजें\?</string>
     <string name="report_description_1">रिपोर्ट आपके सर्वर मॉडरेटर को भेजी जाएगी। आप इस बारे में स्पष्टीकरण दे सकते हैं कि आप इस खाते को रिपोर्ट क्यों कर रहे हैं:</string>