TouchMenu: TouchMenuItem's UnderlineContainer should probably have its own dimen object

The coordinates could wonky otherwise, as TouchMenuItem is an
InputContainer.
Mostly harmless in practice as that UC is its main widget.

frontend/ui/widget/container/underlinecontainer.lua

@@ -12,7 +12,7 @@ local WidgetContainer = require("ui/widget/container/widgetcontainer")
 local UnderlineContainer = WidgetContainer:extend{
     linesize = Size.line.thick,
     padding = Size.padding.tiny,
-    --- @todo shouldn't this default to black instead?
+    -- We default to white to be invisible by default for FocusManager use-cases (only switching to black @ onFocus)
     color = Blitbuffer.COLOR_WHITE,
     vertical_align = "top",
 }

frontend/ui/widget/touchmenu.lua

@@ -145,8 +145,8 @@ function TouchMenuItem:init()
 
     self._underline_container = UnderlineContainer:new{
         vertical_align = "center",
-        dimen = self.dimen,
-        self.item_frame
+        dimen = self.dimen:copy(),
+        self.item_frame,
     }
 
     self[1] = self._underline_container