Reader: Do less work on same-orientation non-gyro rotations (#11297)

Namely, don't recompute layouts, as they do not change.
(The gyro codepaths were already doing something similar.)

* Keep ConfigDialog, FileManagerMenu & ReaderMenu open on rotation.
  (In practice, only ConfigDialog is affected, as *Menu doesn't handle the rotation event.)
* Plugged an instance leak in the aforementioned Menu classes.
* Unify behavior & code with the gyro codepaths.

frontend/apps/reader/modules/readerconfig.lua

@@ -158,11 +158,11 @@ function ReaderConfig:onSwipeShowConfigMenu(ges)
     end
 end
 
+-- For some reason, things are fine and dandy without any of this for rotations, but we need it for actual resizes...
 function ReaderConfig:onSetDimensions(dimen)
-    -- since we cannot redraw config_dialog with new size, we close
-    -- the old one on screen size change
     if self.config_dialog then
-        self.config_dialog:closeDialog()
+        -- init basically calls update & initGesListener and nothing else, which is exactly what we want.
+        self.config_dialog:init()
     end
 end