clean up refreshes

This is a larger clean-up of the refresh situation.
The general shift is that refreshes are now mainly triggered by
the (top-level) widgets when they get shown or closed via UIManager.

All refreshes for the widgets when they are in use were handled by
themselves before. This adds the case of showing/closing.

It is the desired result of not having UIManager:show()/:close()
do (full screen) refreshes on its own.

frontend/apps/reader/modules/readerfont.lua

@@ -149,7 +149,7 @@ function ReaderFont:onSetFontSize(new_size)
     UIManager:show(Notification:new{
         text = T( _("Font size set to %1."), self.font_size),
         timeout = 1,
-    }, nil, nil, "none")
+    })
     self.ui.document:setFontSize(Screen:scaleBySize(new_size))
     self.ui:handleEvent(Event:new("UpdatePos"))