Added a few interfaces for adding/deleting/replacing/saving cheats. The cheats list is guarded by a std::shared_mutex, and would only need a exclusive lock when it's being updated.
I marked the `Execute` function as `const` to avoid accidentally changing the internal state of the cheat on execution, so that execution can be considered a "read" operation which only needs a shared lock.
Whether a cheat is enabled or not is now saved by a special comment line `*citra_enabled`.
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