Experience revamp

[RedRafe]

I'd like to revamp our Experience module with few ideas:

• move it to a shared module under features/ so other scenarios can use it as well
• rework the reward system to be more useful to players & scenarios:
  • give better bonus for character's stats
  • add possibility for scenarios to tweak the steepness of rewards vs effort, whitelist/blacklist effects
• refactor the GUI and make it a little more Factorio-ish

I'll break down this change into 3 steps (possibly 3 PRs) & get review/approval for each individual step before moving to next one, so to make it easy to review.

This first submission only takes care of moving the code from diggy to the common folder of /features. No changes in code logic.