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/menu.lua

@@ -88,7 +88,7 @@ local MenuCloseButton = InputContainer:new{
     overlap_align = "right",
     padding_right = 0,
     menu = nil,
-    dimen = Geom:new{},
+    dimen = nil,
 }
 
 function MenuCloseButton:init()
@@ -544,8 +544,8 @@ local Menu = FocusManager:new{
     -- height will be calculated according to item number if not given
     height = nil,
     header_padding = Size.padding.large,
-    dimen = Geom:new{},
-    item_table = {},
+    dimen = nil,
+    item_table = nil, -- NOT mandatory (will be empty)
     item_shortcuts = {
         "Q", "W", "E", "R", "T", "Y", "U", "I", "O", "P",
         "A", "S", "D", "F", "G", "H", "J", "K", "L", "Del",
@@ -605,9 +605,9 @@ end
 
 function Menu:init()
     self.show_parent = self.show_parent or self
+    self.item_table = self.item_table or {}
     self.item_table_stack = {}
-    self.dimen.w = self.width
-    self.dimen.h = self.height or Screen:getHeight()
+    self.dimen = Geom:new{ w = self.width, h = self.height or Screen:getHeight() }
     if self.dimen.h > Screen:getHeight() or self.dimen.h == nil then
         self.dimen.h = Screen:getHeight()
     end