OpenRA/OpenRA — pull request #7106 by penev92 (2014-12-09 22:21:10+00:00)
Title: 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)?