issue #3851 add cceditbox to wp8

@dumganhar @stammen
I added a usercontrol called EditBox.xaml to the xaml project. It contain a textbox, a 'OK' button, a 'Cancel' button it the top of screen, it cover the screen as Modal window, while clicking the 'OK' button or press the 'enter' key, it set the new text to the label to show, and click 'Cancel' button to return directly.
I used an interface in native component and implemented at xaml to call C#, view the link:http://stackoverflow.com/questions/17304386/how-does-one-make-function-calls-or-trigger-events-from-a-native-component-into as a reference, not using the code as show IME and hide.
Please review it, Thanks.