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-oc/strings.xml

@@ -411,7 +411,6 @@
     <string name="confirmation_domain_unmuted">%s es pas mai rescondut</string>
     <string name="mute_domain_warning">Volètz vertadièrament blocar complètament %s \? Veiretz pas mai de contengut venent d’aqueste domeni, nimai los flux d’actualitat o las notificacion. Vòstres seguidors d’aqueste domeni seràn tirats.</string>
     <string name="mute_domain_warning_dialog_ok">Rescondre tot lo domeni</string>
-    <string name="pref_title_show_notifications_filter">Mostrar lo filtre de las notificacions</string>
     <string name="filter_dialog_whole_word">Mot complèt</string>
     <string name="filter_dialog_whole_word_description">Quand un mot clau o una frasa es solament alfanumeric, serà pas qu’aplicat se correspond al mot complèt</string>
     <string name="pref_title_alway_open_spoiler">Totjorn desplegar las publicacions marcadas amb un avertiment de contengut</string>