itacac: Removed final_setup function, moving its tasks to set_property

and setcaps. Removed rate, channels, difftime, and peak_type members
from itacacpad. Removed old union from data_container struct. Changed
rate from a guint to a gint. Added new GMutex named caps_lock to itacac,
for use when setting caps on pads and ensuring they are consistent with
caps on other pads. Created static function named
get_instrument_pad_pointer for setting up convenience pointers
itacac->*1_itacacpad.