CTextFile: Make Seek more resilient by avoiding calling GetPosition/GetLength when possible.

This prevents ANSI subtitles files with Unix line endings to crash MPC-HC because of Microsoft's standard library getting confused.

This commit fixes #4307.
Cosmetics: Avoid shadowing variables by using better names.

Additionally use AppendFormat when possible.
Add a "Copy to clipboard" feature to the "Play > Filters" menu.

The list of filters is quite useful for debugging purpose.
Replace CFileVersionInfo class by a namespace.

That class had no member variables and only static methods.

Also add some methods to format version string.
About dialog: Add the video devices to the clipboard output.
Auto-change fullscreen monitor mode: Make the settings handling sane.

The settings weren't even initialized on first run. Also simplify a lot the GUI code used to modify those settings.

This commit fixes some cases where the auto-change fullscreen mode monitor settings got corrupted or missing.

Fixes #4298.
Updated LAV Filters to 0.61.2-19-f9cc8f9 (custom build based on 0.61.2-14-85431f4)

Important changes:
 - LAV Splitter: No subtitles were displayed when using the special "Forced subtitles" track created for PGS subtitles (fixes #4326)
 - Updated ffmpeg library
Why "C:\tmp.srt"?
Fix some CPPCheck warnings or mark them as intentional.
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...
Fix: x86 and x64 versions conflicted over file associations.

4f465cf8b81f1da800b1bfc57cc48e317d24fbb5 was incomplete for Vista+.

This commit fixes #4064.