[ivette] replace icon versions with a single compatible one

From the electron-builder doc:
https://www.electron.build/icons.html
- macOS: Icon size should be at least 512x512.
- Windows: Icon size should be at least 256x256.
- Linux: Recommended sizes: 16, 32, 48, 64, 128, 256 (or just 512).

So, using a 512x512 file with default name (icon.png) should work in all
systems and avoid issues with hardcoded differing file names.

ivette/package.json

@@ -99,7 +99,7 @@
           ]
         }
       ],
-      "icon": "static/icon256.png",
+      "icon": "static/icon.png",
       "legalTrademarks": "LGPL-2.1"
     },
     "linux": {
@@ -117,7 +117,7 @@
       "category": "Profiling"
     },
     "mac": {
-      "icon": "static/icon512.png",
+      "icon": "static/icon.png",
       "identity": null,
       "mergeASARs": true,
       "target": [