Add spying to the game
With https://code.wildfiregames.com/rP19175 we can now add spying into the game: you have first to research a tech (available in phase 3, espionage), and then you can bribe a random unit (defined as Bribable in its template) from a chosen player and share its vision during 15s.
There is also an option in the gamesetup to disable this feature, analoguous to the treasure disabling option.
In the current version, only traders (land and naval ones) are bribables.
Reviewed By: Itms, elexis
Differential Revision: https://code.wildfiregames.com/D117