Prevent automatic footer refreshes from drawing the footer on top of stuff that isn't ReaderUI (#6648)

* Prevent refreshing the footer automatically where it doesn't make sense to.

Issue exposed by #6540
Fix #6616

frontend/apps/reader/modules/readerconfig.lua

@@ -81,6 +81,7 @@ end
 
 function ReaderConfig:onShowConfigMenu()
     self.config_dialog = ConfigDialog:new{
+        covers_footer = true, -- hint for UIManager:repaintReaderFooter
         dimen = self.dimen:copy(),
         document = self.document,
         ui = self.ui,