Set the size of the PFSP dialog automatically to 70% of the windows height

This will first compute the size of the window, and set the height of
the dialog to 70% of the window, plus the 140px of decorations.

The width of the dialog is computed based upon the width of the image
plus 33px of decorations. The width is computed by multiplying the
height (70% of the windows height) by the width/height ratio of the
original image.

At the end, once the dialog size computed, the size of the internal
image is updated to reflect the size of the dialog. This is done this
way (instead of setting the size of the dialog to automatic) because of
the placement issues accruing if the size of the dialog box is not
precomputed.

Hacky, but seems to works.