The Uno.Resizetizer extension for icon should be conditionally generated

Current behavior

This icon extension is currently not generated if the icon files are not setup properly, causing hard-to-diagnose errors:

Line 8 in d565936

public class WindowIconGeneratorTask_V0 : Task

We should either generate this extension always, or move this extension to Uno so that it's always available.

this should be addressed when Resizetizer is ported into Uno