Introduce an API for showing a Metro MessageDialog in a MetroWindow

A combination of ideas from both #550 and #657. I created a function on ```MetroWindow``` that allows you to asynchronously show a metro-style MessageBox/Dialog **IN** that MetroWindow.

### Todo list:
- [x] Basic functionality
- [x] Localization friendly
- [x] Tabbing support

### Screenshots
![md2](https://f.cloud.github.com/assets/251501/1458333/84fbae32-4362-11e3-8772-80db188743bb.PNG)

### Code
You can use the new MessageDialog by calling this:
```
///this = a MetroWindow
this.ShowMessageAsync("title","message");
```

### Other stuff
Apart of #744
Fixes #696