Refactor TreeEditor _new_actions and _menu_new_node to avoid hacky eval

[aaronayres35]

Part of #1502

This PR supersedes #1518.

This PR follows the approach outlined here: #1518 (comment)

[aaronayres35]

This try: ... except: ... was necessary since without it the new code changes would cause an exception to be raised if one tried to create a new node for an object with required traits. Previously the hacky eval swallowed this here:

traitsui/traitsui/qt4/tree_editor.py

Lines 1203 to 1219 in a4be4b5

	try:
	eval(
	method_name,
	globals(),
	{
	"object": object,
	"editor": self,
	"node": node,
	"info": info,
	"handler": handler,
	},
	)
	except:
	from traitsui.api import raise_to_debug
	raise_to_debug()
	return

Once we complete the second portion of this fix, namely "You then sit down and think about how you need to change the tree node API so that it can better specify non-class factories (ie. an improvement or alternative to get_add and get_name). You basically need some way to specify the name to display in the menu, and the factory function to call for each type of thing you want to add. There may even be something like this already in the API. The default implementation of this should just do what the current code does, but it should allow developers to have their own tree node classes supply alternatives."
we will want to remove this. At that point a programmer will have all the tools needed to specify how to create a new node that has required arguments, and if they do so in a way that we still can not create a node we should complain and raise an exception. However now, with the machinery not yet in place, we don't want this to fail. This preserves the current behavior.

[aaronayres35]

_node_for_class_name is now no longer called anywhere. Perhaps the method can be deleted? It could be useful in some other case so maybe we want to just leave it, but given it is currently unused, it may make more sense to just delete it and re-add it if a future use case comes about that could make use of it.

[rahulporuri]

_node_for_class_name looks straightforward so if you don't find any users of it, please go ahead and remove it.

[rahulporuri]

This is a nit-picky comment but as long as we are refactoring code in this PR, how about refactoring the _new_actions method to return early if len(add) == 0.

# return early if there are no items to be added in the tree
if len(add) == 0:
    return items

for klass in add:
    ....

Refactoring the code like above makes the code slightly more readable by removing one level of indentation.

Similarly, we can change if add_node is not None to if add_node is None to remove one more level of indentation in the code.

[rahulporuri]

LGTM