Update macOS icon with a native appearance.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

build/icon-macos.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="-100 -100 1024 1024"><defs><linearGradient id="a" x1="0" y1="0" x2="0" y2="1"><stop offset="0" stop-color="#50adff"/><stop offset="1" stop-color="#7877fc"/></linearGradient><mask id="b"><rect x="-100" y="-100" width="1024" height="1024" fill="#fff"/><path d="M566.66 527.25c0 33.03-24.23 60.05-53.84 60.05H260.29c-29.61 0-53.84-27.02-53.84-60.05 0-20.22 9.09-38.2 22.93-49.09l134.37-120c2.5-2.14 5.74 1.31 3.94 4.19l-49.29 98.69c-1.38 2.76.41 6.16 3.25 6.16h191.18c29.61 0 53.83 27.03 53.83 60.05zm0-281.39c0 20.22-9.09 38.2-22.93 49.09l-134.37 120c-2.5 2.14-5.74-1.31-3.94-4.19l49.29-98.69c1.38-2.76-.41-6.16-3.25-6.16H260.29c-29.61 0-53.84-27.02-53.84-60.05s24.23-60.05 53.84-60.05h252.54c29.61 0 53.83 27.02 53.83 60.05z" transform="scale(1.0658112582781456)" fill="#000"/></mask><path id="c" d="M824 257c0-64 0-104-14-141A173 173 0 00708 14C671 0 631 0 567 0H257C193 0 153 0 116 14A173 173 0 0014 116C0 153 0 193 0 257V567c0 64 0 104 14 141A173 173 0 00116 810c37 14 77 14 141 14H567c64 0 104 0 141-14A173 173 0 00810 708c14-37 14-77 14-141Z"/><filter id="d"><feGaussianBlur in="SourceGraphic" stdDeviation="10"/></filter><filter id="e"><feGaussianBlur in="SourceGraphic" stdDeviation="5"/></filter></defs><use href="#c" transform="translate(0 10)" fill-opacity="0.3" filter="url(#d)"/><rect x="120" y="120" width="584" height="584" fill="#fff" /><g filter="url(#e)"><rect x="120" y="120" width="584" height="584" fill="#32497f" mask="url(#b)" transform="translate(0 5)"/></g><use href="#c" mask="url(#b)" fill="url(#a)"/></svg>

package.json

@@ -50,6 +50,7 @@
     "copyright": "©2020 Kandra Labs, Inc.",
     "mac": {
       "category": "public.app-category.productivity",
+      "icon": "build/icon-macos.png",
       "target": [
         {
           "target": "default",
@@ -74,7 +75,7 @@
     },
     "linux": {
       "category": "Chat;GNOME;GTK;Network;InstantMessaging",
-      "icon": "build/icon.icns",
+      "icon": "build/zulip.png",
       "description": "Zulip Desktop Client for Linux",
       "target": [
         "deb",