Updated for 0.15.5, 1.5.1. Didn't push last time.

Fixed the cannot find isDrawing issue in 0.15.4. Not that it matters, since it turns out the field is public again now. Likewise, I used the ObfuscationReflectionHelper system to try and get the field for GuiScreen.buttonList instead of the loop, but it'll still fall back.
Merge branch 'master' of https://github.com/ShaRose/GuiAPI