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-ko/strings.xml

@@ -402,7 +402,6 @@
     <string name="failed_fetch_posts">게시물을 불러오지 못했습니다</string>
     <string name="report_description_1">인스턴스 관리자에게 신고합니다. 이 계정을 신고하려는 이유를 작성하실 수 있습니다:</string>
     <string name="report_description_remote_instance">이 유저는 다른 인스턴스에 속해 있습니다. 그 인스턴스에도 익명으로 신고 내용을 보내시겠습니까\?</string>
-    <string name="pref_title_show_notifications_filter">알림 필터 보이기</string>
     <string name="action_open_as">%s(으)로 열기</string>
     <string name="title_accounts">계정</string>
     <string name="failed_search">검색하지 못했습니다</string>