Add Is8OrLater() and use that in the places we checked for Windows 8.

Fixes ticket #3411.
Build.bat: Support for debug packages.
Put back removed string in mplayerc.eu.rc.txt after 8608c253a918f63586d88d3ed7bb285924957163.
Revise seek-on-keyframe code

It now uses stl structures and algorithms.
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.
MainFrm: remove seek-on-keyframe legacy hack

It breaks accurate keyframe seeking in LAV
Add seek-on-keyframe rounding method selection
AppSettings: seek to nearest keyframe by default
Play > Subtitles menu: Improve the external subtitles naming.