Add pop and setdefault to AppCtxGlobals

[ThiefMaster]

While easy to implement with a custom app_ctx_globals_class class, I believe these two methods are useful enough to add them to the default class.

Especially setdefault makes lots of sense if you e.g. cache something on the app context globals and thus want to do something like this:

cache = g.setdefault('whatever_cache', {})
try:
    return cache['foo']
except KeyError:
    rv = cache['foo'] = get_foo()
    return rv

[untitaker]

Right, thanks!