Add GUT (CLI only) and remove gd-plug

Commit GUT directly to the git repo and remove gd-plug as it is no
longer required to install GUT.

Modify GUT to be used by command-line only.

For example:
```
cp test/.gutconfig.ci.json .gutconfig.json
godot --no-window -s addons/gut/gut_cmdln.gd
```

addons/gut/one_to_many.gd

@@ -0,0 +1,43 @@
+# ------------------------------------------------------------------------------
+# This datastructure represents a simple one-to-many relationship.  It manages
+# a dictionary of value/array pairs.  It ignores duplicates of both the "one"
+# and the "many".
+# ------------------------------------------------------------------------------
+var _items = {}
+
+
+# return the size of _items or the size of an element in _items if "one" was
+# specified.
+func size(one = null):
+	var to_return = 0
+	if one == null:
+		to_return = _items.size()
+	elif _items.has(one):
+		to_return = _items[one].size()
+	return to_return
+
+
+# Add an element to "one" if it does not already exist
+func add(one, many_item):
+	if _items.has(one) and !_items[one].has(many_item):
+		_items[one].append(many_item)
+	else:
+		_items[one] = [many_item]
+
+
+func clear():
+	_items.clear()
+
+
+func has(one, many_item):
+	var to_return = false
+	if _items.has(one):
+		to_return = _items[one].has(many_item)
+	return to_return
+
+
+func to_s():
+	var to_return = ""
+	for key in _items:
+		to_return += str(key, ":  ", _items[key], "\n")
+	return to_return