List of modifiers affecting socketed skill gems

List of modifiers that increase the level of skill gems.

Note that the 3 modifiers with the weighting of 0 can still be obtained:
 * Catarina's: This is the signature mod of Catarina, Master of the Dead; at level 4 she sells items with this modifier
 * Competitor's: Some race starting items have this modifier
 * Haku's: This is the signature mod of Haku, Armourmaster; at level 4 he sells items with this modifier