Move Upgrades and Prerequisites

Moved two interfaces from ```Mods.RA.TraitInterfaces.cs``` to ```Mods.RA.CommonTraitInterfaces.cs```:
 - ITechTreePrerequisite
 - ITechTreeElement

Moved six classes from Mods.RA to Mods.Common:
 - Buildable
 - ProvidesTechPrerequisite
 - ProvidesCustomPrerequisite
 - GlobalUpgradable
 - GlobalUpgradeManager
 - TechTree

Note: I noticed a folder called "Upgrades" in Mods.Common, containing an ```UpgradeManager```. Should I be using that folder for the upgrades, or move them to locations corresponding to their current ones (as I did)?