Replace a writable position() property with a seek() method.

Replace a writable position() property with a seek() method.

Seeking is sometimes asynchronous, and more importantly nearly every
one using a slider for seeking ends up with a binding loop.

Change-Id: I45d92e19b0276c8b97c51c073754d1c9d3dc611e
Reviewed-by: Ling Hu <ling.hu@nokia.com>