Moves the remaining two ServerTraits and 3 out of 4 Modifiers to Mods.Common
Merge pull request #6898 from OpenRA/common-09

Moved remaining two ServerTraits and some Modifiers to Mods.Common
Added IDisabledTrait & rewrote upgrade code using a level-based approach.

	Upgradeable traits are notified whenever an upgrade of their declared types are granted or revoked.  The traits maintain their own internal level counter, which is then used to enable or disable the trait functionality.  A trait can register for multiple upgrade types which then all affect the internal level counter.

	IDisabledTrait for identifying (and filtering) disabled traits
	UpgradableTrait provides an abstract base for traits to support upgrade levels
	Added IDisabledTrait support to GlobalButtonOrderGenerator

	Includes rework by pchote with alterations.