Change Force pixel aspect ratio in advanced clip properties to display

Change Force pixel aspect ratio in advanced clip properties to display
aspect ratio.

Most users understand this better, and computing a pixel aspect when you
just want to force it widescreen is inconvenient. Advanced users who
really want pixel aspect ratio can do the math. This should be backwards
compatible with documents where the pixel aspect ratio was set.

svn path=/trunk/kdenlive/; revision=5029