Rename Callback to Handler

[mottosso]

This PR implements on, and renames all references to callback to handler.

• Rename Callback to Handler #293 (Rename Callback to Handler)

Full backwards compatibility is preserved

Example

Technically, on is simply an alias for register_handler, previously known ad register_callback.

from pyblish import api, util

def on_published(context):
  print("Published!")

api.on("published", on_published)

util.publish()
# Published!

Explicit Support for Graphical User Interfaces.

In addition, this PR implements "weak references" for all event handlers.

This shouldn't have much effect on your day to day use of Pyblish, but will make a difference in a few small corner cases.

For example, consider this.

count = {"#": 0}

class MyObject(object):
    def __init__(self):
        pyblish.api.on("mySignal", self.on_mysignal)

    def on_mysignal(self):
        count["#"] += 1

obj = MyObject()
pyblish.api.emit("mySignal")
assert_equals(count["#"], 1)

del(obj)
pyblish.api.emit("mySignal")
assert_equals(count["#"], 1)

As you would expect, once the object is deleted, the handler stops being called. Prior to this PR however, count["#"] equalled 2.

In practice, this could cause issues when you build a GUI around signals, and register methods within your GUI and then create a new instance of this GUI.

The second instance would reconnect to these signals, leaving the old methods in place, resulting in two calls per method. For each instance of your GUI, the calls would stack up, and off you go.

This PR enables you to build user interfaces that harness the signals produces throughout Pyblish, without worrying about these difficult-to-debug problems.

[mottosso]

This fails on Python 3.3. :( Investigating..