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
Added upgrade support to RepairableBuilding