Reworked mutation_data to make trait vailidity more obvious.

Previously, it seems that everything listed in mutation_data was set as a valid mutation by default.

Instead, I've separated out setting the validity into VALID(true/false). The player won't notice any difference, but it means we can set mutations with prereqs and addons in here that won't be mutated by regular mutagen.

I've added in all the mutations from pl_data, anything that wasn't previously mutable has VALID(false) and from my tests it seems to work correctly in ignoring them when mutating.

I've also taken this opportunity to add in a number of mutations that were listed in categories but weren't mutable by default. After going through them all the number is significantly higher than I thought before. Previously you COULD mutate them, but only if the game selected them via category mutation, you couldn't get them from the random selection.

I initially considered adding in a validity boolean to the traits in pl_data, but since the game currently checks mutation_data when looking at validity I decided on making the change here.