client: Avoid loading the same background plugin again

Load a plugin only if it's the first time or the previously plugin is
different, this avoids loading the same plugin again.
client: Don't link to alsa

Volume control code was moved to a data provider so we don't need this.
mixer: Link to alsa

Forgot to link to alsa here.
dataproviders: Use ${Hawaii_LIBRARIES}
client: Link to QtConfiguration explicitely

It was pulled in by the HawaiiShell dependency but since it doesn't use
it anymore we have to explicitely link to QtConfiguration.