Support custom application icons at multiple sizes

Allow Desktop projects to supply application icon artwork at multiple sizes so that small icons remain clear in window title bars and other operating-system surfaces.

Keep the existing single PNG as a convenient default, with automatic generation of the sizes the developer has not supplied. Add optional PNG artwork for individual sizes and support importing a multi-resolution Windows .ico file. Developers should be able to preview the small versions and see which sizes contain their own artwork and which are generated automatically.

Use the configured artwork consistently when running, debugging, building and publishing the application. Existing projects containing a single application icon should continue to work without requiring changes.