Menu: Don't share the dimen object across Menu instances (!)

The object was never re-assigned, so closing a smaller menu (e.g.,
Calibre metadata search) made the underlying one (e.g., CoverBrowser's
ListMenu) inherit the smaller dimensions...

Instead of creating the object in the Class constructor, create it in the
instance constructor (i.e., :init).

Similar cleanups in other Menu* related classes.

frontend/ui/widget/sortwidget.lua

@@ -149,7 +149,7 @@ local SortWidget = InputContainer:new{
     -- index for the first item to show
     show_page = 1,
     -- table of items to sort
-    item_table = {},
+    item_table = nil, -- mandatory
     callback = nil,
 }