Allow input seeking

The seeking option, `-ss`, can be applied before or after the input filename. Placing it before results in faster encoding if the start point is a significant way through the input file. I'd like this option in the GUI.

When using this option, the reference point for the `-to` option changes to be the start of the clip, rather than the start of the input file. This means the out time will have to be calculated.

The documentation is [here](https://trac.ffmpeg.org/wiki/Seeking).

There are some cases where output seeking (as used now) is preferred, so I guess it should be left as an option. The documentation says:

> Here, the input will be decoded (and discarded) until it reaches the position given by -ss. This will be done very slowly, frame by frame. As of FFmpeg 2.1, the main advantage is that when applying filters to the output stream, the timestamps aren't reset prior to filtering (i.e. when [​burning subtitles into a video](https://trac.ffmpeg.org/wiki/HowToBurnSubtitlesIntoVideo), you don't need to modify the subtitle timestamps), but the drawback is that it will take a lot of time until it finally reaches that time point. The bigger the seeking time is, the longer you will have to wait.