Add a graphical interface for editing inventory triggers

[gerzytet]

The editing interface should do the following:

• Register a /trg i command to open the interface
• Allow removing and placing items, just like a chest!
• When the interface is closed, it should open a prompt in chat asking if they want to save the changes, discard the changes, or keep working (the keep working option enables people to get items from elsewhere, then continue editing once they click the option).

The most tedious part of using trg's inventory triggers isn't coding the trigger, it's usually placing the items, one by one using commands. This is especially true when columns, borders, and large areas of items are used. #187 tried to address this in a simple way, but a more versatile, visual solution would be even better.

Just to be clear, this is NOT the same as #191. That proposes an editing mode for people who trigger an inventory trigger. This issue is about improving on the methods for server operators to permanently edit an inventory trigger.