InlineMediaDownloader: Fix callbacks

Found "thanks" to our great unit tests. There go like 16 hours of my
life.

Conflicts:
	src/InlineMediaDownloader.vala
InlineMediaDownloader: Port to write_pixbuf_async
InlineMediaDownloader: Handle deleted thumbnails

If the thumbnail is not available, we have to unfortunately load the
complete media from disk, create the thumbnail, write that and then use
the it.
InlineMediaDownloader: Fix indentation
InlineMediaDownloader: Catch remaining GErrors
InlineMediaDownloader: Add note if media is not downloadable
Utils: Add write_pixbuf_async function

Since write_to_stream_async is broken, we need to work around that.
Make more classes public

This will be needed anyways to they appear in our generated vapi, which
is needed for testing
InlineMediaDownloader: Use our extern gdk pixbuf functions

We are not depending on the newer valac YET, so async constructors can't
be used.
vapi/gdk-pixbuf: Add variadic argument to save_to_stream
Merge pull request #133 from Lyude1337/master

Add uninstall target to build system
DefaultTimeline: Always show the vertical scrollbar

In "timelines", there *should* always be enough content to show it
anyways, and always showing it remove the weird size-jump when first
initializing a timeline.
UserFilterEntry: Don't forget the screen_name's @
Merge branch 'master' of https://github.com/baedert/corebird
timelines: Move auto scroll login into DefaultTimeline
DefaultTimeline: Don't always decrease the unread_count in mark_seen

If seen was true before, we shouldn't decrease the unread_count.
MentionsTimeline: Code style
timelines: Move more common code into DefaultTimeline
Corebird: Save open windows on quit

Related to #135
HomeTimeline: Don't show notifications in unwanted cases

Before, whenever the stack_count was 1 and auto-scroll was enabled *and*
the ScrollWidget was not scrolled up, we shows a notification.