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/readerback.lua

@@ -142,7 +142,6 @@ function ReaderBack:onBack()
             self.back_resist = true
             UIManager:show(Notification:new{
                 text = _("Location history is empty."),
-                timeout = 2,
             })
             return true
         else