ctl: support extra information to user-defined element set

ctl: support extra information to user-defined element set

In ALSA control feature, information of an element includes extra fields
to type-specific parameters; i.e. dimension. The fields can be extended in
future.

Meanwhile, current APIs to add user-defined element set can not support
such an extended fields. This may cause inconveniences in future.

This commit supports the fields, by changing APIs for element set.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>