DMThreadsPage: Refactor the 2 api calls to twitter

Their callbacks are nearly identical so we better use only one function.
Merge branch 'master' of https://github.com/baedert/corebird
DMThreadsPage: Show Progress indicator row

We normally just use the GtkListBox's placeholder but since there is
already the row to create a new conversation in the list, it's never
shown.
So we just add another row.

This fixes #26(No progress indication) and #88(first download of dms out
of sync).