remove BelowUnits

closes #3670
private readonly elsewhere unused variables
Merge pull request #6456 from Mailaender/remove-belowunits

Removed the BelowUnits trait
Fixed RepairBridgeOrderTargeter & hut logic - it now ignores dangling bridges & rejects bridges under repair

	Closes #4354
fixes crash in minimap generator for some of maps
Merge pull request #6789 from atlimit8/FixRepairBridgeOrderTargeter

Fixed RepairBridgeOrderTargeter & hut logic - it now ignores dangling bridges
Merge pull request #6861 from ihptru/utility-fix

fixes crash in minimap generator for some of maps
Added IDisabledTrait & UpgradableTrait with upgrade levels support

	IDisabledTrait for identifying (and filtering) disabled traits
	UpgradableTrait provides an abstract base for enabling trait
		by upgrade level(s)
	Added IDisabledTrait support to GlobalButtonOrderGenerator