Add option everythingImmutable or immutableByDefault

[Asqiir]

I would like to have an option that makes everything I set with plasma-manager immutable.

Proposed name:
(1) everythingImmutable
(2) forceImmutable
(3) immutableByDefault

Proposed behaviour:

• every option for which a value but no mutability/persistency behaviour was specified: this is now immutable
• every option where persistent is specified: stays persistent
• when something is explicitly marked as immutable = false then (for 1 or 2) fail the build or (3) make not immutable. I don't have a strong intuition on that.

@magnouvean I think that goes with the spirit of your other proposal of changing the override behaviour. What do you think?

[magnouvean]

I reckon option ´3´ makes the most sense since it gives more control to the user and since the implementation would be the most elegant. The only challenge is that at the moment immutability and persistence are incompatible so that will take some work figuring out how we can allow for combining these two in a way that makes sense.