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
MainFrm: Get the source filter from the graph immediately after opening the main file.

Changed all code that depends on the source filter to take advantage of it.
Fixes issues with the wrong source filter being used when multiple source filters are on the graph.
CONTRIBUTING.md: fix forking link.
Updated Czech translation
Spelling correction in Spanish translation.