Sane defaults for the output targets

This wish is probably a bit early, but when the gui-rewrite gets done, this should be considered.

When i open the render dialog, and select one of the targets, their parameters should be magically changed by synfig to sane default settings.

For DV-pal output the image has to be 720x568px framerate is freely changeable, but cold be set to 25 pictures per second.

For gif output the image should be set to 320x240px framerate to 6 frames/sec.

For mpeg output, the framerate is important, because it accepts only certain values. I have not investigated yet, but 25/sec and 30/sec are save defaults i think. Else ffmpeg barfs on the commandline.

I think we should discuss here the sane defaults, to make this watertight and error free.