Clear artifact paths from the cache/settings when needed

[dragos]

There are two levels of caching:

• on disk: .idea/scala.xml, backed by the Idea settings management
• in memory: HydraArtifactsCache

Both need to be cleared if anything changes (for instace, the Hydra version). Perhaps a button in the UI (Refresh) would also be needed, but I'm not sure: if things work properly this shouldn't happen very often).

[dotta]

I think I've experienced a caching issue. I've installed our Hydra plugin and tried to compile. The vanilla Scala compiler was used because I forgot to click the download button. Then, I clicked the download button, the Hydra artifacts have been download, hit rebuild, but the vanilla Scala compiler was still used (instead of Hydra). Finally, after clicking the Download button again a dialog saying that Hydra was already downloaded was displayed and, after hitting the rebuild button again it finally used Hydra to compile the code.