Fixed mouseover support for NotGrid.

[atholbro]

Hey,

With the current NotGrid mouseover extension in Roids, using a mouseover heal macro does not work if you have a target selected. Instead of casting on the target under your mouse cursor, it will instead cast the heal on the person you have targeted. After debugging this a bit, I realized it only applies to NotGrid. Aka if I mouseover a player in the physical world, while targeting another, it works as expected (casts the heal on the mouseover target).

Digging further, I found that the NotGrid mouseover extension is actually not working at all. There are two causes for this:

1. NotGrid loads before Roids, so the current check for arg1 = "NotGrid" will never pass, as NotGrid has already loaded. This means the extension is never enabled.
2. If you force Roids to load first (by renaming it to !Roid-Macros), it still doesn't work, which I'm guessing is due to changes within NotGrid. I didn't dig too far into this, but I'm guessing they changed when or how ConfigUnitFrames() is called.

I went ahead and rewrote the loading/hooking so that it works with the latest version of NotGrid. The changes are basically:

1. Assume that NotGrid will load before Roids, so remove the arg1 check and replace it with a check for the NotGrid global variable.
2. Hook NotGrid:CreateFrames() rather then NotGrid:ConfigUnitFrames() and iterate over the frames as they're stored in a table (NotGrid.UnitFrames). I had to manually hook the function (like done in the ag extension) as it must execute after the original.
3. Added the unregister ADDON_LOADED event, like in the ag extension.

I had first attempted to hook NotGrid:CreateUnitFrame, however this lead to some strange font not set errors that I couldn't figure out, so I went with the loop all frames approach. Otherwise it basically works the same, by hooking the OnEnter/OnLeave scripts.

[DennisWG]

Thank you very much for your contribution! Merged.