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/ui/widget/inputtext.lua

@@ -205,7 +205,6 @@ function InputText:isTextEditable(show_warning)
     if show_warning and not self.is_text_editable then
         UIManager:show(Notification:new{
             text = _("Text may be binary content, and is not editable"),
-            timeout = 2
         })
     end
     return self.is_text_editable