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/readerlink.lua

@@ -508,7 +508,6 @@ function ReaderLink:onClearLocationStack(show_notification)
     if show_notification then
         UIManager:show(Notification:new{
             text = _("Location history cleared."),
-            timeout = 2,
         })
     end
     return true
@@ -758,7 +757,6 @@ function ReaderLink:onGoBackLink(show_notification_if_empty)
     elseif show_notification_if_empty then
         UIManager:show(Notification:new{
             text = _("Location history is empty."),
-            timeout = 2,
         })
     end
 end
@@ -780,7 +778,6 @@ function ReaderLink:onSwipe(arg, ges)
                 -- so the user knows why
                 UIManager:show(Notification:new{
                     text = _("Location history is empty."),
-                    timeout = 2,
                 })
                 return true
             end