Add more methods to OpenGui

[PedroMPagani]

Finish implementation of
https://github.com/brcdev-minecraft/shopgui-api/blob/master/src/main/java/net/brcdev/shopgui/gui/gui/OpenGui.java

maybe add methods such as:

.getButtons(),
.getCategory()

Maybe change that gui or something to have a InventoryHolder implementation so that we can keep track with the .getOpenInventory, and then the inventory holder would bet he one holding htese 2 methods getButtons and category

There could be an event such as ShopOpenGuiEvent, it would contain 3 main fields, Player, OpenGui oldGUi, and OpenGui openingGui
so its also known if the player is switching guis(oldGUi not null)..

Maybe someone else has more addons for the API?