Gold reserves & capped gold

**Is your feature request related to a problem? Please describe.**
Being the first to conquer terrain to obtain the whole player's gold is frustrating.

**Describe the solution you'd like**
(amounts are example)
Starting maximum gold : 250K
Per city level : +250K

Capturing a city gives the gold it holds
Building takes gold from nearest city / default terrain (if nearest city has 0 gold : takes from next city)
Capturing last bit of player's terrain gives at most 250K.

=>
- Shouldn't change early game too much ;
- Can't steal by being last to conquer ;
- City placement is now strategic ;
- Mid game attacks are more strategic ;
- Gives more reason to upgrade city ;
- Unprotected cities in mid+ game is a bigger deal

**Describe alternatives you've considered**
- On factory instead of city : If i'm not mistaken, they will be used for the oil feature. So it would be too valuable
- On a new "bank" building : you'd only need one ?

No files, tests, or entry points are named. Start by tracing how player gold, city capture, terrain capture, and building costs are currently handled; the work is done when the capped-gold and per-city rules are implemented without significantly changing early-game behavior and the stated capture and spending cases are covered.

Written by the indexing model from the issue text.