Fix GetTechModifiedProperty mistake in rP23843 & allow multiple token modifiers.

String-values in modifications fail if the modifications do not apply, as they end up in the "multiply/add" path, following D270 / rP23843.

Also allow multiple token modifiers, so that multiple tokens may be added for example. This becomes order-dependent but the positive outweighs the negative.

Reported By: wowgetoffyourcellphone

Reviewed By: Angen

Differential Revision: https://code.wildfiregames.com/D2894