Ability to have multiple back/previous/next buttons#73

Adding the ability to have multiple back/previous/next buttons in the config would greatly make custom gui’s easier to use with ShopGui+!

Right now you are only able to use Dummy buttons for “Back” inside shop menus which is a pain to add to each shop!

Being able to use SLOTS in buttons would be really useful

slots: 
   - NUMBER
   - NUMBER
   - NUMBER