app: the preedit_end mark must be created with right gravity.

With left gravity, the mark is moved back 1 character to the left of the
cursor, whereas with right gravity, it is right after newly-inserted
text. This resulted sometimes into non-deleted preedit characters.
I don't understand all the subtleties since the problem only occurred
for the first character of a text layer, at least in my tests.
Anyway now it should be good.

app/tools/gimptexttool-editor.c

@@ -1511,7 +1511,7 @@ gimp_text_tool_im_preedit_changed (GtkIMContext *context,
                                         gtk_text_buffer_get_insert (buffer));
       text_tool->preedit_end = gtk_text_buffer_create_mark (buffer,
                                                             "preedit-end",
-                                                            &iter, TRUE);
+                                                            &iter, FALSE);
 
       /* Move the cursor to the expected location. */
       gtk_text_buffer_get_iter_at_mark (buffer, &iter, text_tool->preedit_start);