GtkTextView: add some FIXME and TODO comments
Add boolean return value for gtk_text_tag_table_add()

https://bugzilla.gnome.org/show_bug.cgi?id=614717
gtk_text_buffer_create_tag(): return NULL on failure

https://bugzilla.gnome.org/show_bug.cgi?id=614717
Fix gtk_text_iter_forward_to_tag_toggle() for end iter

The function must return TRUE only if there is a tag toggle _after_ the
iter, not _at_ the iter. So for the end iter, the function must always
return FALSE.

Add also unit tests for gtk_text_iter_forward_to_tag_toggle().

https://bugzilla.gnome.org/show_bug.cgi?id=691266
textbuffer: fix selection replacement when pasting

It is more logical to first delete the selection and then pasting the
text. When the selection and the text contain tags, the new behavior is
more natural.

A segfault in paste_from_buffer() is also avoided. The segfault occurs when
the text to paste is deleted because it is the selection.

https://bugzilla.gnome.org/show_bug.cgi?id=339539