This is now stored separately for playback can capture in the playback
and capture sub-objects respectively.

This commit also renames the onLog member of mal_context_config to
logCallback for consistency with mal_device_config callback naming
convention.

There is now only a single API for initializing a device config which
is mal_device_config_init() and takes the device type as it's only
parameter. By default this will use the device's native configuration.

The device ID is also set in the device config. The next commits will
be changing mal_device_init() to remove the device type and ID from the
parameter list since they are now specified in the config.

This swaps the order of the input and output pointers so that it's
consistent with the order used by memcpy().

This removes the two separate callbacks for sending and receiving data
to/from the device to a unified callback that's used for both input and
output. The new callback takes a pointer to both an input and output
buffer. When the device is opened in playback mode the input pointer
will be set to null. Likewise the output pointer will be set to null
for capture devices. Both input and output pointers will be non-null
for full-duplex devices.

This change makes a device run it's own loop with a timer to better
simulate an actual device.