Remove AccountInfoWidget
Account: banner_url is nullable
Utils: Close stream in download_async
Account: Close stream in update_avatar
PixbufButton: Scale background to widget size
style.css: Add pixbuf-button-round class

Which will be used on round pixbuf buttons
PixbufButton: Make round buttons possible
AvatarBannerWidget: Use buttons for avatar and banner
AvatarBannerWidget: Remove old properties/variables
Show filechoosers when avatar/banner get clicked
ComposeTweetWindow: Mention the retweeters screen_name when replying

Fixes #239
ComposeTweetWindow: Don't add own screen_name when replying...

... to own tweets.

Fixes #241
TweetInfoPage: Protected accounts are able to see their own tweets
ComposeTweetWindow: Don't add too many spaces

Since we now don't add a screen name in the case of a user replying to
themselves, we need to be more careful about adding spaces before and/or
after screen names.

Fixes #245
Add CropWidget
Add ImageCropDialog
examples: Add crop dialog example
Update PO files
DMThreadsPage: The progress spinner should not be activatable
DMThreadsPage: Don't show the spinner if not connected