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.
Add some helper functions to check if a string matches an ISO 639 code or language.
Play > Subtitles menu: Improve the external subtitles naming.

Show the subtitle filename and detect the language whenever possible.
Sync mplayer.eu.rc.txt after 3c79306a0eca973bcb72061531b680d0d185d809.

Missing string in 8608c253a918f63586d88d3ed7bb285924957163 reappeared here...
DSUtil: Replace inline assembler by a compiler intrinsic.

This commits works around AStyle being fool by the non-standard __asm keyword.
Fix some implicit int64 to int32 conversion.
custom_messages.iss: updated Chinese Simplified translation.
Remove unused code.
Update Changelog.txt.
Updated Basque translation (ticket #2890).
Updated French translation.
Updated Polish translation.
Updated simplified Chinese translation.
Updated German translation.
ShockwaveGraph: restore process volume on exit

Applies to Vista and later.
Fixes #2852