Avoid warnings with no pixbuf in UmCropArea

https://bugzilla.gnome.org/show_bug.cgi?id=710874
Set a nice default size on CheeseWidget

As per mockups:

https://raw.github.com/gnome-design-team/gnome-mockups/master/avatar-chooser/avatar-chooser.png

https://bugzilla.gnome.org/show_bug.cgi?id=710874
Style the CheeseAvatarChooser button bar

Style the button bar underneath the video widget properly.

As per mockups:
https://raw.github.com/gnome-design-team/gnome-mockups/master/avatar-chooser/avatar-chooser.png

https://bugzilla.gnome.org/show_bug.cgi?id=710874
Rename the CheeseAvatarChooser buttons

As per mockups:
https://raw.github.com/gnome-design-team/gnome-mockups/master/avatar-chooser/avatar-chooser.png

https://bugzilla.gnome.org/show_bug.cgi?id=710874
Add margins to CheeseAvatarChooser

https://bugzilla.gnome.org/show_bug.cgi?id=710874