Use correct size for window icon

This PR fixes #1242. A new control `MultiFrameImage` replaces the `Image` control in the `IconTemplate`, selecting the best matching icon frame from the provided icon source.

To reproduce the fix, remove all image formats from [`mahapps.metro.logo2.ico`](https://github.com/MahApps/MahApps.Metro/blob/master/samples/MetroDemo/mahapps.metro.logo2.ico) except 16x16 and 32x32 (I didn't want to include the modified icon file in this PR :wink:) and start the demo application.