Fix string leaks in deserialize vfuncs

When an object contains char * properties, its deserialize function will
overwrite these string values without freeing them first. They are not
necessarily NULL as they can have (default) values set upon construction,
so they need to be g_free'd before being overwritten.

For example, IBusEngineDesc::longname, IBusEngineDesc::description,
IBusEngineDesc::language, ... are all set to "" upon construction (instead of
NULL), so the corresponding IBusEngineDesc fields must be freed before
being overwritten during deserialization.

This commit introduces a ibus_g_variant_get_child_string() to do this
and set the string value.

This leak was reported by valgrind:

==22163== 59 bytes in 59 blocks are definitely lost in loss record 1,633 of 2,720
==22163==    at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==22163==    by 0x56DFDF2: g_malloc (gmem.c:97)
==22163==    by 0x56E011D: g_malloc_n (gmem.c:338)
==22163==    by 0x56FAFCC: g_strdup (gstrfuncs.c:356)
==22163==    by 0x546CDB8: g_value_dup_string (gvaluetypes.c:1136)
==22163==    by 0x4E43A98: ibus_engine_desc_set_property (ibusenginedesc.c:385)
==22163==    by 0x5446CA3: object_set_property (gobject.c:1378)
==22163==    by 0x54484BB: g_object_constructor (gobject.c:2020)
==22163==    by 0x4E24564: ibus_object_constructor (ibusobject.c:111)
==22163==    by 0x5447577: g_object_new_with_custom_constructor (gobject.c:1645)
==22163==    by 0x5447787: g_object_new_internal (gobject.c:1722)
==22163==    by 0x5447C75: g_object_newv (gobject.c:1868)
==22163==    by 0x544737A: g_object_new (gobject.c:1568)
==22163==    by 0x4E2521C: ibus_serializable_deserialize (ibusserializable.c:292)
==22163==    by 0x4E480E0: ibus_component_deserialize (ibuscomponent.c:408)
==22163==    by 0x4E2523E: ibus_serializable_deserialize (ibusserializable.c:294)
==22163==    by 0x4E4C1D1: ibus_registry_deserialize (ibusregistry.c:202)
==22163==    by 0x4E4C95C: ibus_registry_load_cache_file (ibusregistry.c:362)
==22163==    by 0x4E4C6A4: ibus_registry_load_cache (ibusregistry.c:302)
==22163==    by 0x40EBCD: bus_ibus_impl_registry_init (ibusimpl.c:1871)
==22163==    by 0x40C1D4: bus_ibus_impl_init (ibusimpl.c:424)
==22163==    by 0x5460FCD: g_type_create_instance (gtype.c:1868)
==22163==    by 0x5448466: g_object_constructor (gobject.c:2006)
==22163==    by 0x4E24564: ibus_object_constructor (ibusobject.c:111)
==22163==    by 0x5447577: g_object_new_with_custom_constructor (gobject.c:1645)
==22163==    by 0x5447787: g_object_new_internal (gobject.c:1722)
==22163==    by 0x54483C1: g_object_new_valist (gobject.c:1980)
==22163==    by 0x54473C9: g_object_new (gobject.c:1571)
==22163==    by 0x40E94A: bus_ibus_impl_get_default (ibusimpl.c:1807)
==22163==    by 0x41D880: bus_server_init (server.c:100)
==22163==    by 0x420E91: main (main.c:236)

BUG=http://code.google.com/p/ibus/issues/detail?id=1712

Review URL: https://codereview.appspot.com/104850043
Patch from Christophe Fergeau <cfergeau@redhat.com>.