plug-ins: add a "Load Defaults" button to reset print resolution...

... in the print plug-in.
The resolution set during printing may be different from the print size,
for various reason (home printing vs. printshop for instance). The print
plug-in's resolution is saved and reused from a non-persistent parasite,
thus masking actual intended print resolution. Adding this button allows
to reset this temporary value to the intended resolution at any time.