No more hardcoding :)

Required to be able to copy/clone a paramspec

The expected results is now different due to fixes in babls CIE Lab
in babl commit 0dba0608bd2cfe69453834c1ec87b7a2be13d2dd

The expected results is now different due to fixes in babls CIE Lab
in babl commit 0dba0608bd2cfe69453834c1ec87b7a2be13d2dd

fix a typo and a memory leak

The original scale-size requires to pass both height and width.
scale-size-keepaspect will calculate one from the other, preserving
the original aspect ratio if only one is specified.
If no dimensions are specified, keeps original dimensions.

Peeks inside at the content-type to determine loader to use.
Tested for png and jpg files.

Relies on the extension just like file paths. Not ideal, but
tricky to do much better without fetching URI and checking
content-type (could be done for HTTP for instance).

Special string property which is a URI/URL (incl dataURL).
Annotated differently so that user interfaces can show a
dialog particular for picking URIs.