clock: Add gst_clock_abort_wait() function
Submitted by Carlos Rafael Giani
As mentioned in https://bugzilla.gnome.org/show_bug.cgi?id=749391#c10 , the newly introduced gst_clock_wait_for_sync() function issues a blocking wait. If this function is used inside elements for example, it is important to be able to abort the wait, for example during a PAUSED->READY state change. A gst_clock_abort_wait() function is needed that aborts the blocking wait.
To that end, also extend the description of gst_clock_wait_for_sync(). It shall return FALSE if either a timeout happened or the wait was aborted.