Small XtmRefreshRate memory leak in settings dialog

Steps to reproduce:

• Run valgrind --leak-check=full --show-leak-kinds=definite xfce4-taskmanager
• Open the settings dialog in the task manager
• Close the dialog
• Close the task manager

Leak report:

==12879== 16 bytes in 1 blocks are definitely lost in loss record 2,361 of 12,491
==12879==    at 0x4848953: calloc (vg_replace_malloc.c:1595)
==12879==    by 0x59892BA: g_malloc0 (gmem.c:163)
==12879==    by 0x11DF0F: builder_bind_combobox (settings-dialog.c:99)
==12879==    by 0x11E58A: xtm_settings_dialog_new (settings-dialog.c:198)
==12879==    by 0x11E963: xtm_settings_dialog_run (settings-dialog.c:242)
==12879==    by 0x113238: show_settings_dialog (process-window.c:235)
==12879==    by 0x58F6B72: UnknownInlinedFun (gclosure.c:895)
==12879==    by 0x58F6B72: signal_emit_valist_unlocked (gsignal.c:3516)
==12879==    by 0x58F6F4F: g_signal_emit_by_name (gsignal.c:3716)
==12879==    by 0x58F6B72: UnknownInlinedFun (gclosure.c:895)
==12879==    by 0x58F6B72: signal_emit_valist_unlocked (gsignal.c:3516)
==12879==    by 0x58F6C76: g_signal_emit_valist (gsignal.c:3355)
==12879==    by 0x58F6D33: g_signal_emit (gsignal.c:3675)
==12879==    by 0x4B627E8: ??? (in /usr/lib/libgtk-3.so.0.2407.32)

Analysis:

This XtmRefreshRate data doesn't appear to be cleaned up:

99	refresh_rate = g_new0 (XtmRefreshRate, 1);

Version: a23c5c30