XSTUDIO_PLUGIN_PATH on Windows splits on the wrong character

[HughMacdonald]

When splitting XSTUDIO_PLUGIN_PATH, it uses ':' on all platforms, instead of using ';' on Windows. (ref:

xstudio/src/plugin_manager/src/plugin_manager_actor.cpp

Line 33 in 57f2468

for (const auto &p : xstudio::utility::split(plugin_path, ':')) {

)

I've got a PR that I've created to fix this issue