Handle stacking status effects, modifiers, some related fixes.

Continuation of work on status effects, following rP23448.

Allow techs to modify properties.
Enable different behaviour when status effects would stack, such as extending the duration or stacking the effects.
Fix GUI issue with more than 5 status effects.
Improve tooltips.
Let status effects show a different tooltip for the attacker and the receiver.

Patch by: Freagarach

Reviewed By: wraitii, Angen (on an older differential).

Differential Revision: https://code.wildfiregames.com/D2296