[Settings] added option to always use the included/portable AviSynth build (disabled by default)
[Update]   improved AviSynth portable support for external tools
[Log]      added AviSynth+ detection


git-svn-id: http://svn.code.sf.net/p/megui/code@2444 a21b8521-4df1-4ed3-b48c-cff5bc73dcb3