Comment 22 for bug 1349838

I have applied the diff of 1002, but it generates build warnings.

xpad-text-buffer.c: In function ‘xpad_text_buffer_freeze_undo’:
xpad-text-buffer.c:422:47: warning: passing argument 1 of ‘gtk_source_buffer_begin_not_undoable_action’ from incompatible pointer type [-Wincompatible-pointer-types]
  gtk_source_buffer_begin_not_undoable_action (buffer);
                                               ^~~~~~
In file included from /usr/include/gtksourceview-3.0/gtksourceview/gtksource.h:27,
                 from xpad-text-buffer.c:26:
/usr/include/gtksourceview-3.0/gtksourceview/gtksourcebuffer.h:189:72: note: expected ‘GtkSourceBuffer *’ {aka ‘struct _GtkSourceBuffer *’} but argument is of type ‘XpadTextBuffer *’ {aka ‘struct XpadTextBuffer *’}
 void gtk_source_buffer_begin_not_undoable_action (GtkSourceBuffer *buffer);
                                                       ~~~~~~~~~~~~~~~~~^~~~~~
xpad-text-buffer.c: In function ‘xpad_text_buffer_thaw_undo’:
xpad-text-buffer.c:428:45: warning: passing argument 1 of ‘gtk_source_buffer_end_not_undoable_action’ from incompatible pointer type [-Wincompatible-pointer-types]
  gtk_source_buffer_end_not_undoable_action (buffer);
                                             ^~~~~~
In file included from /usr/include/gtksourceview-3.0/gtksourceview/gtksource.h:27,
                 from xpad-text-buffer.c:26:
/usr/include/gtksourceview-3.0/gtksourceview/gtksourcebuffer.h:192:70: note: expected ‘GtkSourceBuffer *’ {aka ‘struct _GtkSourceBuffer *’} but argument is of type ‘XpadTextBuffer *’ {aka ‘struct XpadTextBuffer *’}
 void gtk_source_buffer_end_not_undoable_action (GtkSourceBuffer *buffer);