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-cy/strings.xml

@@ -307,7 +307,6 @@
     <string name="notification_clear_text">Ydych chi\'n siŵr eich bod chi am glirio\'ch holl hysbysiadau\'n barhaol\?</string>
     <string name="error_multimedia_size_limit">Ni all ffeiliau fideo a sain fod yn fwy na %s MB.</string>
     <string name="url_domain_notifier">%s (🔗 %s)</string>
-    <string name="pref_title_show_notifications_filter">Dangos yr hidlydd hysbysiadau</string>
     <string name="error_following_hashtag_format">Gwall wrth ddilyn #%s</string>
     <string name="error_unfollowing_hashtag_format">Gwall wrth ddad-ddilyn #%s</string>
     <string name="action_unmute_domain">Dad-dewi %s</string>
@@ -732,4 +731,4 @@
     <string name="about_copied">Copïwyd fersiwn a gwybodaeth dyfais</string>
     <string name="list_exclusive_label">Cuddio o\'r ffrwd cartref</string>
     <string name="error_media_playback">Methodd chwarae: %s</string>
-</resources>
+</resources>