Allow guides with off-canvas position since we can now view and work
outside the canvas.

The guide deletion interaction does not change too much, except you
don't delete a guide because it's dropped off-canvas but off the display
viewport area instead.

The XCF format is technically unchanged as PROP_GUIDES was already
expecting an int32 coordinate (with max value at GIMP_MAX_IMAGE_SIZE,
way below uint32 max anyway). Yet our code and XCF docs was explicitly
asking to ignore negative coordinate guides, which makes a change in
behavior for the XCF parser, hence a new version XCF 15. Loading will
still ignore negative position guides (even also bigger than image
dimension guides now) for XCF 14 and below, but will now accept any
position for XCF 15 and above.