IndicateIndicator's leaks strings when indicators are added, removed, modified

The issue is in the g_variant_get() calls in libindicate/listener.c's proxy_signal_cb() call. The "IndicatorAdded", "IndicatorRemoved", and "IndicatorModified" handlers all pull type or property strings with g_variant_get(variant,"s",&str) but don't free the strings.

We don't hold onto any of these pointers long-term, so we'd be better off using g_variant_get(variant,"&s",&const_str) instead.