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

globals.d.ts

@@ -5,6 +5,7 @@ import { Options, Variable as VariableType } from 'types/variable';
 
 declare global {
     var useTheme: (filePath: string) => void;
+    var getSystrayItems: () => string;
     var isWindowVisible: (windowName: string) => boolean;
     var globalWeatherVar: VariableType<Weather>;
     var options: Options;