Improvements to steam workshop support & in-game modding plus crash fix.

[1.40]

1. Fixed rare crash bug when starting a second game after quitting one where media spin events had recently been used.
2. Changed the perception UI so that the values are labelled as +- 50% in all cases instead of sometimes being +-100%. (UI only change).
3. Fixed bug where you could implement some policies that should be hidden if you searched for them from the main UI.
4. Improved mod support to prevent confusion between workshop-installed mods and local mods you are developing.
5. Added support for modding situations.