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-gl/strings.xml

@@ -128,7 +128,6 @@
     <string name="label_duration">Duración</string>
     <string name="create_poll_title">Enquisa</string>
     <string name="pref_title_enable_swipe_for_tabs">Activar xestos de desprazamento para moverse entre lapelas</string>
-    <string name="pref_title_show_notifications_filter">Mostrar filtro das notificacións</string>
     <string name="failed_search">Fallou a busca</string>
     <string name="title_accounts">Contas</string>
     <string name="report_description_remote_instance">A conta pertence a outro servidor. Queres enviar unha copia anónima da denuncia alí tamén\?</string>