Use | as path separator in all path variables

[jesper-friis]

Suggestion from @francescalb

Why not have the same separator in all? Also, one could envision plugins being available on the internet somewhere (for the future)?

Would be smart to switch to change | as default path separator now, while keeping : for backward compatibility (with deprecation warnings) for some releases. This would allow for a smooth transition to online plugins.

By the way, how would we ensure safe evaluation of an online plugin?