[RFC] Icon scaling

[thoemmi]

This PR adds the property IconScalingMode to MetroWindow. It's forwarded to MultiFrameImage.MultiFrameImageMode to set the icon scaling. There are two possible values:

• MultiFrameImageMode.ScaleDownLargerFrame
  
  It takes the smallest frame which from the icon which has equal or larger size as the window's icon template. The frame is scaled down it's larger.
  
  
• MultiFrameImageMode.NoScaleSmallerFrame
  
  It takes the largest frame from the window which has equal or smaller size than the window's icon template. The frame is rendered centered if it's smaller.
  
  

I've added two menu items to the Window menu in the demo application.

I created a special icon for reproduction, but didn't want include it in this PR. Just replace Mahapps.Metro.Tests\mahapps.metro.logo2.ico with the icon in this ZIP file.

RFC: I'm not satisfied with the naming of the properties and the enum. If you have a better suggestion I would be happy to rename them.

This fixes #2242.

[punker76]

@thoemmi I want to merge this PR with these namings...

For MetroWindow

IconMultiFrameScalingMode -> type IconMultiFrameScalingModes

For MultiFrameImage

MultiFrameScalingMode -> type IconMultiFrameScalingModes

public enum IconMultiFrameScalingModes {
    ScaleDownLargerFrame,
    NoScaleSmallerFrame,
}

[thoemmi]

I recommend staying with singular IconMultiFrameScalingMode. See Enumeration Type Naming Guidelines: as long as it's not a bitfield (annotated with FlagsAttribute), it should use a singular name, because there's always only one mode active.

The enum is called MultiFrameImageMode because it is used by MultiFrameImage. That MetroWindow is using the control to display the icon is secondary. Therefore I hesitate to prefix the enum's name with Icon.