Replace materialdesignicons.com with pictogrammers.com

[drfuzzyness]

Some time ago, https://materialdesignicons.com started redirecting to https://pictogrammers.com/library/mdi/ which is causing the URL copying feature to stop working. This changes the copy URL to use the new https://pictogrammers.com/library/mdi/ URL.

This doesn't update the actual icon font or meta.json and some icons might have been deprecated in the time since the last update.

[Garulf]

Thank you for your contribution. I am in the process of updating and porting all my plugins to https://github.com/garulf/pyflowlauncher

Once I get to it I'll try and update this plugin if possible.