Added the ability to define custom icons for the system tray. (#408)

* Added the ability to define custom icons for the system tray.

* Add placeholder icon if icon is not defined.

* Update themes

themes/catppuccin_macchiato.json

@@ -340,5 +340,6 @@
   "theme.bar.buttons.modules.submap.icon_background": "#24273a",
   "theme.bar.menus.menu.network.switch.enabled": "#c6a0f6",
   "theme.bar.menus.menu.network.switch.disabled": "#363a4f",
-  "theme.bar.menus.menu.network.switch.puck": "#494d64"
+  "theme.bar.menus.menu.network.switch.puck": "#494d64",
+  "theme.bar.buttons.systray.customIcon": "#cad3f5"
 }