Input access locking, part 3 (final).

Input access locking, part 3 (final).

Enforce access to input_item_t through input_item_{G,S}et*()
Unlock mutexes before calling these functions
Free strings returned by input_item_Get*()

New functions added:

char * input_item_GetURI( input_item_t  * )
void input_item_SetURI( input_item_t * , char * )

mtime_t input_item_GetDuration( input_item_t * )
void input_item_SetDuration( input_item_t * , mtime_t )

to access input_item_t->psz_uri and input_item_t->i_duration