Fix small get_res_icon_path() widget leak

`horizons.gui.util.get_res_icon_path()` does the same thing as as
`horizons.gui.widgets.imagebuton`: aks fife to create an `Icon()` to
see if it's a valid image path. But other than the `ImageButton`,
`get_res_icon_path()` did forget to call `hide()` on that `Icon`. So the
`Icon` stayed alive, even though unused.

We now call `hide()` on that `Icon`.

horizons/gui/util.py

@@ -125,7 +125,7 @@ def get_res_icon_path(res, size=32, greyscale=False, full_path=True):
 		icon_path = icon_path + '{res:03d}.png'.format(res=res)
 
 	try:
-		Icon(image=icon_path)
+		Icon(image=icon_path).hide()
 	except fife.NotFound: # ImageManager: image not found, use placeholder or die
 		if res == 'placeholder':
 			raise Exception('Image not found: {icon_path}'.format(icon_path=icon_path))