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-sl/strings.xml

@@ -407,7 +407,6 @@
     <string name="confirmation_domain_unmuted">Domena %s odrita</string>
     <string name="mute_domain_warning">Ali ste prepričani, da želite blokirati vse iz domene %s\? Vsebine iz te domene ne boste videli v nobeni javni časovnici ali v obvestilih. Vaši sledilci iz te domene bodo odstranjeni.</string>
     <string name="mute_domain_warning_dialog_ok">Skrij celotno domeno</string>
-    <string name="pref_title_show_notifications_filter">Pokaži filtre obvestil</string>
     <string name="filter_dialog_whole_word">Cela beseda</string>
     <string name="filter_dialog_whole_word_description">Če je ključna beseda ali fraza samo alfanumerična, bo uporabljena le, če se ujema s celotno besedo</string>
     <string name="title_accounts">Računi</string>