Add a way to control style handling in the export module.

It is now possible to select whether the style module are to be
appended to the current picture history or as before just replacing
the corresponding module in the history.
Spots IOP: legacy_params(): adapt for latest mask changes

Shame on me, this should have landed before 1.6,
it was lying in load-raw-raw-data branch (PR #619) since at least Aug 5,
but i totally forgot about this particular commit...

Consequences:
for an image with spot v1, opened from mid summer until now:
	upgrade to v2 = results in broken mask
Now:
	upgrade to v2 = ok...
Merge branch 'export-style-dup-module'

* export-style-dup-module:
  Add a way to control style handling in the export module.
New routine dt_view_image_only_expose to display only an image.

This will be used by the print module.
Rename some lightroom types to avoid name clashes with the print module.
Initial revision of the print module.

This is a new module. It requires CUPS to send the print job to the
printer queue. The module offers a way to add some margin, select
the paper size, the print queue. It also support a printer/paper ICC
profile.
Add support for replace/append style in print module.