Add MateMixerDeviceSwitch and MateMixerStreamSwitch classes
pulse: Use MateMixerDeviceSwitch and MateMixerStreamSwitch
oss: Use MateMixerStreamSwitch
alsa: Always assign switches to streams and never to devices
alsa: Use MateMixerStreamSwitch

Also introduce alsa-types.h and move instance typedefs there to fix cross referencing introduced by this change
Split MateMixerSwitchRole into MateMixerDeviceSwitchRole and MateMixerStreamSwitchRole
Return MateMixerDeviceSwitch and MateMixerStreamSwitch from functions when feasible and include docs of the new classes
Bump version