Changing the plugin icons

[Rasbats]

https://github.com/Rasbats/frcurrents_pi
Steps to reproduce:

1. Uninstall the plugin
2. Change the icons (SVG)
3. Reinstall, using an imported tarball with new icons.

Icons remain the same as previous.

The cache is not updated when the plugin is uninstalled. Old icons listed.

Even removing the cache items does not change the icons, after a fresh uninstall/install.

Windows 10.0, OpenCPN 5.8.4, API 1.16.