mainwindow: Actually use DAR (not PAR) to set the viewer's display aspect ratio

Using the pixel aspect ratio to set the viewer's (display) aspect ratio
made no sense. We had a nice unused method for that exact purpose.

This also happens to fix bug #722003
clipmediaprops: Use proper names for checkbutton widgets
clipmediaprops: Prevent setting an invalid framerate from a misdetected clip

Fixes bug #721888
Add a comment for translators to clarify "Pixel aspect ratio"

Also update the French translation