ITimeline: Add error handling in load_newest_internal
Move from Soup.SessionAsync to Soup.Session
DMPage: Load older messages when scrolling up

This fixes issue #28
HomeTimeline: Mark retweeted tweets as retweeted

If a tweet was retweeted from the TweetInfoPage, it is now also
maked as retweeted in the HomeTimeline.
MentionsTimeline: Also mark RTs as RTs

... just like we did in HomeTimeline.
Also, remove the leftover debugging output from the previous commit.
ImageDialog: Fix save dialog

Fixes #29