Add Picture.FromSymbol to the standard library

Add Picture.FromSymbol to the standard library: convert any named system symbol (the same SystemSymbol catalogue that Button's symbol picker uses) into a Picture value directly in code, in the colour and size you choose. The returned picture works everywhere a Picture works - a TreeView node icon, an ImageViewer, a canvas, a clipboard export - so the whole symbol catalogue becomes usable as artwork, not just as Button decoration.