Clean up auto-cast related code

[Kenpachi2k13]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Currently the auto-cast related code is a mess.

• We have map_session_data->state.autocast which is used to temporarily flag a skill as auto-cast for skills cast by bAutoSpell* bonuses or by Auto Shadow Spell. It's also (mis)used to prevent damage reflection from infinite looping.
• We have map_session_data->state.abra_flag which isn't actually used. It's just set when casting Abracadabra or Improvised Song, but then never used for validation.
• We have map_session_data->skillitem and map_session_data->skillitemlv which hold the auto-cast skill's ID and level. map_session_data->skillitem is misused to validate auto-casting all over the code.
• As of late we have the itemskill() script command related states and helper variables.
• Unsetting/validating the auto-cast data in the right place currently is more likely gambling.

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I spent some days in getting into the subject and cleaned it up as far as possible.

• I added an enumeration for auto-cast types to be able to handle each one separately and use those types for validation instead of the auto-cast skill's ID.
• I added a structure to unify all auto-cast related data in one place.
• I added a function which validates the plausibility of auto-cast related data.
• I revised where and when pc_autocast_clear() (former pc_itemskill_clear()) gets called.
• I removed obsolete code parts.
• I made skills cast by Improvised Song ignore all requirements.

Closes #1211