Revise seek-on-keyframe code

It now uses stl structures and algorithms.
To nearest rounding is used instead of to lower.
When dragging seekbar thumb, it 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.