libxml has a XML_PARSE_HUGE parsing option, which can be turned on with
the RSVG_HANDLE_FLAG_UNLIMITED option in librsvg.
We cannot just set this option by default because it is a security
hazard as a maliciously malformed SVG could use this to consume too much
memory.

Instead, let's just propose the option interactively when we fail to
create a rsvg handle. Unfortunately right now we can't single out this
specific error because librsvg actually returns an unrelated (false
positive created by the huge data) error. So we just propose the option
for any kind of handle creation failure.

Furthermore, the option is only available on interactive plug-in calls
so far. In particular, the PDB API doesn't have an option allowing a
script writer to run "file-svg-load" with the huge data option.

As for the thumbnail API, it is never meant to be used interactively and
not really as a common script function, so it won't have the huge data
option either.