memory leak + dangling pointer use in indicator_application_dispose()

[Charles Kerr]

There are a couple of memory bugs with clearing out out directories from our icon search path.

1. indicator_application_dispose() calls g_hash_table_get_keys() N times and leaks them all.

2. indicator_application_dispose() passes each of the hashtable's keys (owned by the hash) to function theme_dir_unref(key), which removes from the hashtable but then continues to use the now-dangling key.