Cleanup: De-duplicate dynamic enum function used for setting GP brush icons
It turned out that there were slight differences between these two versions (namely, where they searched for the list of active brushes), but it's not too much of a stretch to use the same code for this with a few minor tweaks, as per this commit.
* Colors have labels, so previews don't need to be as big, GP_BOX_SIZE set to 32. * More padding between items, they are too close to each other at the moment GP_BOX_GAP set to 24. * Round boxes are a bit less round to match the rest of the UI 0.2f (before used 0.4f) * Cursor should not change to color picker. We are performing an action (selecting an item), not picking UI colors per se.