[gtk] Non-fast-forward update to branch wip/baedert/widget-add-style-class

[Timm Bäder <baedert src gnome org>]

The branch 'wip/baedert/widget-add-style-class' was changed in a way that was not a fast-forward update.
NOTE: This may cause problems for people pulling from the branch. For more information,
please see:

 https://wiki.gnome.org/Git/Help/NonFastForward

Commits removed from the branch:

  81d62f1... widget: Add gtk_widget_add_style_class

Commits added to the branch:

  d6a643f... widget: Add gtk_widget_{add,remove}_style_class