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/strings.xml

@@ -610,8 +610,8 @@
     <string name="error_list_load">Error loading lists</string>
     <string name="list_exclusive_label">Hide from the home timeline</string>
     <string name="list">List</string>
-    <string name="notifications_clear">Clear</string>
-    <string name="notifications_apply_filter">Filter</string>
+    <string name="notifications_clear">Delete notifications</string>
+    <string name="notifications_apply_filter">Filter notifications</string>
     <string name="filter_apply">Apply</string>
 
     <string name="compose_shortcut_long_label">Compose post</string>
@@ -670,7 +670,6 @@
     <string name="title_accounts">Accounts</string>
     <string name="failed_search">Failed to search</string>
 
-    <string name="pref_title_show_notifications_filter">Show Notifications filter</string>
     <string name="pref_title_enable_swipe_for_tabs">Enable swipe gesture to switch between tabs</string>
     <string name="pref_title_show_stat_inline">Show post statistics in timeline</string>