New images should obviously still default to the template pixel density
(defaulting to 300.0 PPI when no specific template is selected).

Loaded images though should use a more conservative default of 72 PPI,
first because this is what other software defaults to when no density is
set (so we should keep consistent when possible), and this is also what
the Exif standard (I checked both last version 2.32, and older 2.3)
recommends when no resolution is set.

Technically we differentiate a loaded from a newly created image by
whether or not an imported_file has been set. Of course, any explicitly
set resolution will always override whatever default.