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-sv/strings.xml

@@ -418,7 +418,6 @@
     <string name="failed_fetch_posts">Misslyckades att hämta status</string>
     <string name="report_description_1">Anmälan kommer att skickas till din servermoderator. Du kan beskriva varför du anmäler kontot nedan:</string>
     <string name="report_description_remote_instance">Kontot är från en annan server. Skicka en anonym kopia av anmälan dit också\?</string>
-    <string name="pref_title_show_notifications_filter">Visa notifikationsfilter</string>
     <string name="filter_dialog_whole_word">Helt ord</string>
     <string name="filter_dialog_whole_word_description">När nyckelordet eller frasen enbart är alfanumerisk, appliceras den om den matchar hela ordet</string>
     <string name="pref_title_alway_open_spoiler">Expandera alltid inlägg med innehållsvarningar</string>