Improvement improvements!

- More accurate improvement stat previews for edge cases (e.g. removing Forest on Forest + Lumber Mill)
- 'Create improvement' uniques can create roads and remove features

Set initial screen color on Desktop so it's not black-to-blue

Fixed rare AI City State Influence crash

By SomeTroglodyte:
- Ruleset validator: Tilesets
- Minor Mod Manager fixes (mods having dashes in their repo name not shown right away)