Notification: closed by any event, but not consuming it

Make Notification have toast=true, and UIManager deal
specifically with such widget:
a "toast" widget gets closed by any event, and let
the event be handled by a lower widget.
This should allow us to not wait or tap to get rid
of a notification, and just go on with what we're
doing.
Also make them have a default timeout of 2s,
used with all existing ones.

frontend/apps/reader/modules/readerrolling.lua

@@ -1406,7 +1406,6 @@ function ReaderRolling:onToggleReadingOrder()
     end
     UIManager:show(Notification:new{
         text = is_rtl and _("RTL page turning.") or _("LTR page turning."),
-        timeout = 2.5,
     })
     return true
 end