Alsa: fix crash when detecting devices.

Some old versions of Alsa crash when snd_device_name_hint(-1, ...) is
called. This patch works around the problem by iterating manually over all
the available sound cards.

Change-Id: Ic380a371acc15013d137553ff30d68bed5af664e
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
Move win32 and Alsa audio backends into plugins.

Change-Id: I9835cf5ee97900569f26421a19543b485e933051
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>