reorganize some stuff

modules/core_widgets/core_widgets.otmod

@@ -10,5 +10,4 @@ Module
     require 'uibutton'
     require 'uilabel'
     require 'uicombobox'
-    require 'uipopupmenu'
-    return true
+    require 'uipopupmenu'

modules/core_widgets/uicheckbox.lua

@@ -0,0 +1,8 @@
+UICheckBox = extends(UIWidget)
+
+function UICheckBox.create()
+  local checkbox = UICheckBox.internalCreate()
+  checkbox:setFocusable(false)
+  checkbox:setAlign(AlignLeft)
+  return checkbox
+end