Introduce an API for showing a Metro MessageDialog in a MetroWindow

[AzureKitsune]

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:

• Basic functionality
• Localization friendly
• Tabbing support

Screenshots

Code

You can use the new MessageDialog by calling this:

///this = a MetroWindow
this.ShowMessageAsync("title","message");

Other stuff

Apart of #744
Fixes #696

[spiritdead]

@Amrykid remember for use the tasks in framework 4, the person need the nuggets specific for this file!

[AzureKitsune]

You only need the nuget file if you want the "await" keyword in .NET 4. The demo is using the API without it.