Disable refresh button while API call is in progress

[timruffles]

Quick POC of doing this without any significant changes to the code. It'd be good to get comments on some thoughts I had about how the API call is handled in the model and view (below)

The current API code makes this tricky to do particularly well:

• auto refresh and manual refresh could be simultaneously called
• the callbacks will be fired whether it's auto or manual

It'd be nice to create a Promise object or similar to return from API calls, which the caller could connect to, e.g:

promise = self.controller.sync_api()
self.refresh.setEnabled(False)
promise.on_settled(lambda: self.refresh.setEnabled(True))

[redshiftzero]

hey @timruffles - yeah the current API calling is a bit tricky right now (a larger refactor is upcoming in #100 to make it easier to work with), but this diff looks great