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-is/strings.xml

@@ -396,7 +396,6 @@
     <string name="report_description_remote_instance">Notandaaðgangurinn er af öðrum vefþjóni. Á einnig að senda nafnlaust afrit af kærunni þangað\?</string>
     <string name="title_accounts">Notandaaðgangar</string>
     <string name="failed_search">Tókst ekki að leita</string>
-    <string name="pref_title_show_notifications_filter">Birta tilkynningasíu</string>
     <string name="create_poll_title">Athuga</string>
     <string name="duration_5_min">5 mínútur</string>
     <string name="duration_30_min">30 mínútur</string>