Keyframe Dock, when hit JMP column title , an assert was fired.

(synfigstudio:11914): Gtk-CRITICAL **: gtk_tree_model_sort_set_sort_column_id: assertion `header != NULL' failed

Was caused by "column->set_sort_column(COLUMNID_JUMP);" ( KeyframeTree::KeyframeTree() L 113 ) tricks to detect JMP column in
KeyframeTree::on_event(GdkEvent *event) with if((ColumnID)column->get_sort_column_id() == COLUMNID_JUMP)

Now, the test to detect the JMP column is based on the column title : _("Jump")