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/readerfont.lua

@@ -211,7 +211,6 @@ function ReaderFont:onSetFontSize(new_size)
     self.ui:handleEvent(Event:new("UpdatePos"))
     UIManager:show(Notification:new{
         text = T( _("Font size set to %1."), self.font_size),
-        timeout = 2,
     })
 
     return true
@@ -221,7 +220,6 @@ function ReaderFont:onSetLineSpace(space)
     self.line_space_percent = math.min(200, math.max(50, space))
     UIManager:show(Notification:new{
         text = T( _("Line spacing set to %1%."), self.line_space_percent),
-        timeout = 2,
     })
     self.ui.document:setInterlineSpacePercent(self.line_space_percent)
     self.ui:handleEvent(Event:new("UpdatePos"))
@@ -269,7 +267,6 @@ function ReaderFont:onSetFontGamma(gamma)
     local gamma_level = self.ui.document:getGammaLevel()
     UIManager:show(Notification:new{
         text = T( _("Font gamma set to %1."), gamma_level),
-        timeout = 2,
     })
     self.ui:handleEvent(Event:new("RedrawCurrentView"))
     return true
@@ -293,7 +290,6 @@ function ReaderFont:setFont(face)
         self.font_face = face
         UIManager:show(Notification:new{
             text = T( _("Redrawing with font %1."), face),
-            timeout = 2,
         })
 
         self.ui.document:setFontFace(face)