Additional styles for CtrlInventoryGridEx

[peter-kish]

As described in this issue:

What do you think of adding an option for field highlighted style? say for two colors, one if the item is swappable/droppable (green by default) and one for if its not swappable (red by default) and making the cursor a cross if the item isn't swappable?

[kyle-wannacott]

Slight edit, I think possibly three colors:
Droppable (default): (semi-transparent black ,or semi-white)
Swappable: (semi-transparent green)
Not-swappable: (semi-transparent red)