core: Icons add searching custom file paths

[CMurtagh-LGTM]

Fixes: #147

This is basically just @Kruziikrel13's QML code ported to C++.

This works, haven't had any bad tray icons using this code

  IconImage {
    source: {
      let icon = root.item.icon;
      if (icon.includes("?path=")) {
        const [name, path] = icon.split("?path=");
        icon = `file://${path}/${name.slice(name.lastIndexOf("/") + 1)}`;
      }
      return icon;
    }
    asynchronous: true
    implicitSize: ShellGlobals.sizes.font.normal * 1.1
  }

The main difference is that Kruziikrel13 does this before qt checks the QQuickPixmapCache enabling use of the QML's cache but is overriding the default icon.