Icons overload general review and code cleaning (refactoring)

This patch is fixing some icons overload. I've also performed a code
refactoring that all check always follows the same structure based on
embedded switches (always 2) for each kind of spell family. It helps a lot
to keep consistency while having to revert a check (for instance, checking
overloading between spells having different family must be performed twice,
once per case).

Although there are many Icons overload fixed, icons overload may still
exist in the game.

Also please note that I've not only fixed the reported overload but
a complete set of overload (by fixing directly on the SpellIconID where
it was possible - checking in the DBC).

The list of fixed overload:

https://www.getmangos.eu/issue.php?issueid=243
https://www.getmangos.eu/issue.php?issueid=244
https://www.getmangos.eu/issue.php?issueid=245
https://www.getmangos.eu/issue.php?issueid=246
https://www.getmangos.eu/issue.php?issueid=247
https://www.getmangos.eu/issue.php?issueid=248
https://www.getmangos.eu/issue.php?issueid=249
https://www.getmangos.eu/issue.php?issueid=250
https://www.getmangos.eu/issue.php?issueid=251
https://www.getmangos.eu/issue.php?issueid=252
https://www.getmangos.eu/issue.php?issueid=253
https://www.getmangos.eu/issue.php?issueid=254
https://www.getmangos.eu/issue.php?issueid=255
https://www.getmangos.eu/issue.php?issueid=256
https://www.getmangos.eu/issue.php?issueid=257
https://www.getmangos.eu/issue.php?issueid=258
https://www.getmangos.eu/issue.php?issueid=259
https://www.getmangos.eu/issue.php?issueid=260
https://www.getmangos.eu/issue.php?issueid=261
https://www.getmangos.eu/issue.php?issueid=262
https://www.getmangos.eu/issue.php?issueid=263
https://www.getmangos.eu/issue.php?issueid=264
https://www.getmangos.eu/issue.php?issueid=265
https://www.getmangos.eu/issue.php?issueid=266