plugins: g_thread_init() is deprecated

For glib >= 2.32.0
plugins: Prefer g_str_has_prefix() over strstr

Most of the tested strings should short. Still there is no need to look
for prefixes in the middle of those string.
plugins: Fix commit 5e66fa43bd
gravatar: Don't leak GParamSpec

The GParamSpecPool in the plugin registry takes a reference of the param
spec so it needs to be unrefed in any case, not just in error.

Under contract for Canonical Ltd.

This fixes https://bugzilla.gnome.org/show_bug.cgi?id=680435
Bump to 0.1.20