Improve tooltip comparison

[nanderson11]

Improve the comparison tooltip so that it reacts to pressing shift instantly, instead of requiring you to leave the item frame and come back.

[nanderson11]

You can see how AtlasLoot does it here: https://github.com/Hoizame/AtlasLootClassic/blob/8e99341e4e779328460bf7684c0d5b22ce50ddf1/AtlasLootClassic/Button/Item_type.lua