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/readercropping.lua

@@ -64,6 +64,20 @@ function PageCropDialog:init()
     }
 end
 
+function PageCropDialog:onCloseWidget()
+    UIManager:setDirty(nil, function()
+        return "partial", self[1].dimen:combine(self[2].dimen)
+    end)
+    return true
+end
+
+function PageCropDialog:onShow()
+    UIManager:setDirty(self, function()
+        return "partial", self[1].dimen:combine(self[2].dimen)
+    end)
+    return true
+end
+
 local ReaderCropping = InputContainer:new{}
 
 function ReaderCropping:onPageCrop(mode)