fix broken search context brought about by !69
Since !69 (merged), find_next_button and find_previous_button (that are no longer hidden along with the findbar) don't work after switching panes. These messages can be seen (before Meld freezes):
CRITICAL Gtk: real_set_mark: assertion '_gtk_text_iter_get_btree (where) == tree' failed
CRITICAL Gtk: real_set_mark: assertion '_gtk_text_iter_get_btree (where) == tree' failed
CRITICAL Gtk: gtk_text_buffer_set_mark: assertion 'gtk_text_iter_get_buffer (iter) == buffer' failed
I don't know Python or GTK, but this commit seems to solve this problem for me. I've tried to create a merge request from my cloned repository, but it didn't work.