Fix: Swimming event does not trigger without breath meter

[KevinTyrrell]

ItemRack has always had a major flaw that it cannot detect swimming without going underwater. This is inconvenient for the user. Here's a fix. I can make a pull request if need be.

• /itemrack opt
• -> Events
• -> Swimming

Set the following field:

• Event Trigger: SPELL_UPDATE_USABLE

There is no event for treading water, thus we have to hook into SPELL_UPDATE_USABLE. IsSwimming() is checked in line one of the default script, therefore no harm is done besides very minor overhead for SPELL_UPDATE_USABLE events.