Fix leak in Status Manager dialog

Fix the leak of the GObject wrapper
```
580 (72 direct, 508 indirect) bytes in 3 blocks are definitely lost in loss record 30,987 of 34,328
at 0x484386F: malloc (vg_replace_malloc.c:393)
by 0x4980168: g_malloc (gmem.c:130)
by 0x4997AB5: g_slice_alloc (gslice.c:1074)
by 0x49980EC: g_slice_alloc0 (gslice.c:1100)
by 0x4DDF04B: g_type_create_instance (gtype.c:1913)
by 0x4DC4C1F: g_object_new_internal (gobject.c:2228)
by 0x4DC6247: g_object_new_with_properties (gobject.c:2391)
by 0x4DC6FF0: g_object_new (gobject.c:2037)
by 0x48B79DF: pidgin_status_manager_add (pidginstatusmanager.c:138)
by 0x48B7B2A: pidgin_status_manager_populate_helper (pidginstatusmanager.c:158)
by 0x496801F: g_list_foreach (glist.c:1092)
by 0x48B7B74: pidgin_status_manager_refresh (pidginstatusmanager.c:169)
```

Testing Done:
Ran valgrind, opened Statuses and an editor for a status, and the leak was gone.

Reviewed at https://reviews.imfreedom.org/r/2301/