Issue #4194: resolution passes from 300ppi to 72ppi after saving.
We are running gimp_image_set_imported_file() when saving, with NULL to drop the tie with the imported file. Let's only change the default resolutions when we actually set an imported file. Also set the `resolution_set` flag even when it doesn't change the current values (hence no undo or signals), for instance setting from 300 to 300 PPI. In such case, even though nothing changes, the resolution has to still be considered as explicitly set. In the reporter case, any one of these 2 fixes is enough. See also commit fef9b1d2 (set to 72 PPI as default for imported files only) and commit a8f552da (set imported file to NULL).