systemclock: add OTHER clock type
systemclock: add OTHER clock type

Add an OTHER clock type so that subclasses are able to mark themselves as
using some other clock source than the realtime or monotonic clock.

jpakkane/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,lovebug356/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,krieger-od/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,surround-io/gstreamer,krieger-od/gstreamer,lovebug356/gstreamer,krieger-od/gstreamer,cfoch/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,lubosz/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,Lachann/gstreamer,cfoch/gstreamer,lubosz/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,Lachann/gstreamer,surround-io/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,collects/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,justinjoy/gstreamer,StreamUtils/gstreamer,justinjoy/gstreamer,shelsonjava/gstreamer,cfoch/gstreamer,surround-io/gstreamer,collects/gstreamer,jpxiong/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,centricular/gstreamer,surround-io/gstreamer,lubosz/gstreamer,mparis/gstreamer,mparis/gstreamer,justinjoy/gstreamer,cablelabs/gstreamer,justinjoy/gstreamer,collects/gstreamer,ensonic/gstreamer,krichter722/gstreamer,krieger-od/gstreamer,StreamUtils/gstreamer,collects/gstreamer,lovebug356/gstreamer,Lachann/gstreamer,cfoch/gstreamer,krichter722/gstreamer,justinjoy/gstreamer,shelsonjava/gstreamer,cfoch/gstreamer,mparis/gstreamer,ensonic/gstreamer,centricular/gstreamer,jpakkane/gstreamer,lubosz/gstreamer,collects/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer,mparis/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer