ImageLabel: Don't crash if no icon is set
ImageDialog: Remove the 'open original' menu item

It never worked and the user can really just click the link in the
tweet/dm/whatever.
DMPage: Reset lowest_id in on_join

This makes scrolling up actually behave correctly.
Timelines: Scroll up when activated but already active

Also, don't use switch_page when switching to the first timeline
on startup.
Profile-/TweetInfoPage: Switch to TweetInfoPage when tweet is activated
HomeTimeline: Don't wrongly mark tweet as unseen

If the user wrote the tweet himself and auto_scroll_on_new_tweets
was active, the new tweet was wrongly marked as unseen, then on
scrolling up marked as seen.
MentionsTimeline: Mark mention as seen if the user replied to it
InlineMediaDownloader: Don't recreate sessions all the time
Add a setting to control the maximum inline media download size

Since we're not only using 'normal' services(i.e. Twitter, twitpic,
etc.) to download/display inline media, we need to make sure we're not
downloading huge images.

Closes #48
Timelines: Fix code duplication

All of our timelines use the same tweet activation logic and all have
the same placeholder so we can just do all that in DefaultTimeline.
DefaultTimeline: Give the last activated widget focus on join
corebird.desktop: Capitalize application name
SettingsDialog: max media download size is in megabytes
SettingsDialog: Fix label for auto-scroll-on-new-tweets setting

Oops.
style: Make the text-button hover style less noisy
TweetInfoPage: Make the user name a link to his profile page
TweetListEntry: Shrink the space betweet user name and text
ITimeline: Use better error handling for Twitter api calls
TweetListEntry: Don't let retweet_button and conv_image overlap

This happened whenever the user retweeted a tweet that was also part of
a conversation.
TweetListEntry: Fix hover_box margin-right