Revise seek-on-keyframe code

It now uses stl structures and algorithms.
To nearest rounding is used instead of to lower.
When dragging the seekbar thumb, the thumb now sticks to keyframes
if seek-on-keyframe is enabled.
PlayerSeekbar: handle doubleclick event

To even the numbers of ups and downs.
PlayerSeekbar: remove m_rtPosReal

It just duplicates m_rtPos in current code.