#150 Disabling spells and abilities

[ashsaver]

Various spells and abilities prevent playing of other spells and abilities (Cease Fire, Meddling Mage, Pithing Needle, etc).

Now, a discussion of how best to implement this is probably needed.

There are several ways to implement this functionality:
-modify the "playable" tag in the activated-ability on a card to "nowhere" - this prevents playing the card, but can be overridden unless the priority for the effect is the lowest. (Magic rules say the "can't effect" wins here.)
-constrain playability by action-constraints and a "cannot-play" tag. The problem here is that there's no action that encompasses all spells/abilities, not even pay-mana.
-constrain playability by something similar to action-constraints. This requires new code and probably is less ideal.

Option 1 seems the easiest but I have the feeling there's something wrong there. Option 2 is "ideal" but currently unimplementable without changing all of the card structures. Option 3 requires extra structures period.

Let me know what you think.