Create Workforce variables & effects

[LordPeter]

Workforce is a new concept replacing earlier simplified concepts.
Each pop contributes a certain amount of workforce of a certain type, and in addition, each empty province & built holding contributes a bit of simple_labour.
There are 2 types: simple_labour & craftsmanship.
Exploiting resources requires simple_labour, while producing refined and finalized goods requires craftmanship.
Depending on tech or other circumstances, pop values might be multiplied by a modifier.
Ideally, a province should have enough workforce to produce/exploit tradegoods up to the province limit, which is entirely independent of workforce.

• implement workforce variable
• write effect to calculate it from pops & holdings
• cross-link it with resource spawn events

[LordPeter]

NOTE: This is closed as well, was already linked with spawn chocie events (as resources themselves do not care about workforce, their "parent" spawns do). Will need to let git know about this being closed.