calculate widgetcontainer dimension if not given

frontend/ui/widget/container/widgetcontainer.lua

@@ -10,6 +10,12 @@ function WidgetContainer:init()
 	if not self.dimen then
 		self.dimen = Geom:new{}
 	end
+	if not self.dimen.w then
+		self.dimen.w = self[1].getSize().w
+	end
+	if not self.dimen.h then
+		self.dimen.h = self[1].getSize().h
+	end
 end
 
 function WidgetContainer:getSize()