plug-ins: properly check widget class holding tag data.

Some tag data is hold in GtkEntry, other in GtkTextView, which are not
parent to each other. Previous checks were wrong and resulted in
"invalid cast from 'GtkEntry' to 'GtkTextView'" WARNINGs (followed by
many CRITICALs because of this first error).

Also properly free the data returned by gtk_text_buffer_get_text() which
is allocated (unlike strings returned by gtk_entry_get_text() which must
not be freed).