"note: Using rexx instead of qt4."
Yes, I've understand that, thats I wrote native app. GUI that will use ffmpeg and/or mencoder is in my hummble opinion better choice, as such solution will cover menu more formats and codecs that AviDemux right now support. And as is for sure more easy to manage fresh updates of ffmpeg and mencoder than fresh ports of AviDemux, it will give more new formats and codecs in future.
On the other side, what will be possible to do with this GUI frontend I'm not sure, except simple cut/paste video those tools allows resizing, resampling, put titles in video, put pictures in video, combine video and audio, and many more.
I'm for now only concerned about speed and sensitivity of slider because REXX. When You move slider around, how fast video window can update. And it is possible to play video with variable speed inside this window ?
And how You show content of video inside this window, using mplayer ? Or this is just static image which is extracted by ffmpeg ?