Fix a leak in settings list

Fixes a leak for plugins that have settings:
```
48 (32 direct, 16 indirect) bytes in 1 blocks are definitely lost in loss record 12,560 of 29,780
at 0x4A231EF: g_type_create_instance (gtype.c:1909)
by 0x4A08C1F: g_object_new_internal (gobject.c:2228)
by 0x4A0A247: g_object_new_with_properties (gobject.c:2391)
by 0x4A0AFF0: g_object_new (gobject.c:2037)
by 0x4C64DA7: gtk_size_group_new (gtksizegroup.c:300)
by 0x487356A: gplugin_gtk_plugin_settings_list_refresh (gplugin-gtk-plugin-settings-list.c:492)
by 0x4873CBA: gplugin_gtk_plugin_settings_list_set_settings (gplugin-gtk-plugin-settings-list.c:662)
by 0x4870DEE: gplugin_gtk_plugin_page_update_settings_list (gplugin-gtk-plugin-page.c:98)
by 0x48717DD: gplugin_gtk_plugin_page_set_plugin (gplugin-gtk-plugin-page.c:394)
by 0x48750EF: gplugin_gtk_view_show_plugin (gplugin-gtk-view.c:567)
by 0x4873FD0: gplugin_gtk_view_row_activated (gplugin-gtk-view.c:82)
by 0x49F8A64: g_cclosure_marshal_VOID__OBJECTv (gmarshal.c:1910)
```

Testing Done:
Opened viewer in valgrind, went to settings plugin's info, and leak was no longer there.

Reviewed at https://reviews.imfreedom.org/r/2305/