Specifically the methods related to requesting an upload slot and uploading a file.
Also show a progress indicator while a file is being uploaded.
Updates #161
It contained only `overrides` and some HTTP upload code was in other
modules.
Current thinking concerning overrides:
Usage of `overrides`, while useful in certain cases, should in general
be discouraged, since it's in essence "monkey patching" which makes it
more difficult to know whats executing at runtime and more difficult to
refactor.
Splitting modules up between XEPs is not always that useful. Some XEPs,
like HTTP Upload (and MAM comes to mind) have their functionality spread
out over single and group chats (and pubsub) and might for practical
purposes be considered "core" enough to not try and keep them in
separate modules (which inevitably requires overrides or a fundamentally
rethinking the architecture).
Where splitting code between modules makes a lot of sense is in keeping
Backbone Models and Views separate (so that alternative view libraries
like Vue could be used) and probably in keeping Single chats, MUC,
PubSub and MIX separate.
updates #161
