Added tag "[itemaction] set slot material amount"

[JustusJG]

Issue: #415

[rockyhawk64]

Thanks for the PR. Just flagging why this isn't merged yet, which is due to the maintenance scope this opens up. A basic set action for just material and amount would immediately have people wanting custom model data, item model, display name, lore, and so on. Without those this feature has limited use for most servers, but supporting it means maintaining a full item builder inside CommandPanels.

The current internal item builder that creates panels is technically able to do this with something like the following code snippet, but it adds necessary custom persistent data into the items that will break stacking with other Non-CommandPanels items in players inventories:

if (action.equalsIgnoreCase("set")) {
    InventoryPanelBuilder tempBuilder = new InventoryPanelBuilder(ctx, player);
    ItemBuilder builder = new ItemBuilder(ctx, tempBuilder);
    item = builder.buildItem(panel,
        ((InventoryPanel)panel).getItems().get(args[2]));
}

Not closing this yet but wanted to flag why it hasn't been merged for now.