Avoid some full refreshes on Kindle (#3315)

"partial" refresh causes a full (without black flash) refresh on
Kindle (which uses REAGL mode for partial refresh). This causes a
full redraw of widgets, which is a bit distracting with some of them:
- dictquicklookup: when showing next definition
- infomessage: when displaying a new one (Wikipedia Save as epub)

Also fix bottom menu, that even when closed, would still register
bottom area as dirty: this would cause top menu navigation to
cause a full partial refresh, only noticable on Kindle.

frontend/apps/reader/modules/readerconfig.lua

@@ -105,6 +105,7 @@ end
 
 function ReaderConfig:onCloseCallback()
     self.last_panel_index = self.config_dialog.panel_index
+    self.config_dialog = nil
     self.ui:handleEvent(Event:new("RestoreHinting"))
 end