modding: Negative tile damage cannot heal more than max health

docs: Added more documentation for uniques

chore: Slight readability improvements to Battle.kt

chore: Split NextTurnAutomation into Religion, Trade, and Diplomacy automation files

chore: RulesetValidator readability

Mods with atlases that reference non-existent files will no longer cause crashes

chore: Updated LibGDX to 1.12.1

By SomeTroglodyte:
- Reorg SpecialistAllocationTable 
- Refactor the "loading image" from Multiplayer into a reusable Widget 
- Remove obsolete filtering Unique declaration 

Improved settler AI  - By tuvus

By Framonti:
- add tests for CityFounder 
- fix flaky tests by properly assigning pop to work on specific tile 

By SeventhM:
- Fix Multiplayer Specatator being unable to move the screen 
- Fix free stat buildings not giving unique stat buildings in certain cases