Viewer: add option to zoom to print size using file's ppi.

[ubacher]

It is handy to evaluate what an image will look like at "printed" size.
This requires only that the pixels/inch of the user's display be entered in preferences.
Using the x and y resolution stored in the exif record one can then display the image at the
size at which it would be "printed" (displayed)

( Photoshop has this under VIEW)

My key suggestion:Ctrl F1 ( close to Ctrl 1)