Abort unfinished XHR requests when calling reloadList

Previously, clicking e.g. "unread" and then clicking "newest"
before the first request had finished and while the spinner
was still visible, the page would first show unread items
and a second later show the newest items instead.

Also, the "Load list error" alert is now only shown when
errorThrown isn't empty. Previously, the alert would be shown every
time the user refreshes the page while there's still an XHR running.
Abort unfinished XHR when going to/returning from sources
Don't refresh items twice after returning from sources

This change triggers the onhashchange handler without sending
the reloadList ajax request, instead of triggering it only after the
request has been made and the items have already just been refreshed.
Merge pull request #287 from seanrand/reloadList-fix

Fix issues with refreshing the items list and slow ajax requests