Add axis label and unit to plots

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Remove per channel gain control

There is only one gain control that is used both channels.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Add support for gain selection

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Don't try to update combobox entry from the combobox callback

If the select_gain/select_led functions are called from the combobox
'changed' signal callback do not try to set change the item as it should
already be the selected one.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>