Multiple bug fixes - Spigot 1.15 support

It feels like a lifetime since I last released a new update, but here are finally the few bugfixes that have been requested over the last few months...

As you've probably guessed since I haven't updated it for months now, I'm usually rather busy but do plan to keep developping plugins whenever I have some free time!

The plugin has now moved to 1.15, as did GriefPrevention. If you want to use it for 1.14 servers, you should remain on the old version of the GriefPrevention modified jar provided in an earlier release, as this modified one will not be compatible with spigot 1.14...