Stack Exp creature flag mechanism can be improved

Not necessarily a bug, but I propose a change to how stack exp monster flags abilities work.

The current mechanism, for example, No Retaliation stack exp ability:
- Check if a stack (creature type) has No Retaliation stack exp ability
- If a stack has No Retaliation stack exp ability, cancel its No Retaliation flag -> This creates a compatibility issue
- Check the current stack exp rank and see if No Retaliation stack exp ability should be activated
- If the stack reaches a certain rank and No Retaliation stack exp ability should be activated, give No Retaliation flag to the stack

The problem here is, in Amethyst by Feanor and many existing scripts, we hooked @43D5C7 to manage temporary stats changes and creature flags. This timing is earlier than the timing of stack exp manipulation and thus stack exp overrides any other changes. For example:
- A hero has a modded artifact called BUCKLER_OF_THE_BEELZEBUB, gives all the friendly units No Retaliation @43D5C7
- Since one of the unit has also No Retaliation stack exp ability, this given No Retaliation is cancel by stack exp ability code
- This unit does not meet the rank to activate No Retaliation ability from stack exp, thus it ends up with losing the No Retaliation ability from BUCKLER_OF_THE_BEELZEBUB.

In Short:
No Retaliation stack exp can possibly cancel No Retaliation ability obtained from other sources.

My proposal:
Stack Exp should not cancel the creature flag in its implementation. It should check if a flag exists and skip adding the same flag instead.