Linedef Executor delay field can't use math prefixes

Basically what's in the title, being unable to use ++/-- like some other fields allows is quite inconvenient when you want to increment or decrement multiple delays at once, like you could in binary by changing the backside sector floor and ceiling height.

Edit: turns out the same issue applies to the sector trigger tag field, so if this type of field also exists in other properties, I suppose it needs some improvement to support the math prefixes.