WP8 backbutton fix

This pull request fixes the following issues:

1. Back button event on device does not occur on cocos2d-x rendering thread. I added a new BackButtonEvent to be processed on the rendering thread. 

2. Added C# MessageBox to XAML so CCMessageBox will now display using native WP8 MessageBox which is not available from C++ code.

3. Refactored recently added CCEditBox code to fix broken WP8 C++ builds and to match the style of the other delegates implemented in MainPage.xaml.cs.

4. Moved CPP sample xaml and cs files to cocos2dx/platform/wp8-xaml to allow reuse and reduce bugs by not introducing changes in different copies of the same file.

5. Fixed broken wp8-xaml cpp template broken by previous checkins by community developers.

6. Added CopyTemplateProject to update wp8-xaml cpp template files if they have changed.

7. Increase FPS by about 25% by fixing size of synchronized texture. Improves performance on HD phones like the Lumia 920.