Add previews in the single image import dialog

TODO: some file types, rotation and testing ...
Use dt logo when no preview can be generated
density curve preview Squashed commit of the following
modify filmstrip to keep the active image in the center
Handle empty tags + fix memleak
added base line for better orientation in bauhaus sliders.

next step: make them colored for gradient sliders and such.
added same thing to bauhaus popup, too.
fix for trac issue #395: do not clamp final output of sharpen module
factored out slider baseline drawing to port gradients.
gradient sliders in bauhaus. graduatednd is guinnea pig.
toned down gradient sliders a bit.
Revert "Add check for not adding a background job when the same"

This reverts commit 482d49046f67b35448d5b5300ff584ea163c3bb1.
french ui updated + corrections for "tag" translation
Updated rawspeed from upstream, r419 brings camera definitions
for Nikon D800 and Sony SLT-A57.
Always keep an uneven count of columns to prevent jumpings and
always keep centered images centered in strip.
Fixes so the filmstrip cant be scrolled out of range which
prevents jumpings...
Add 2 columns to always fill the start/end of filmstrip
with half images if not a whole one can fit..
Always write changes before applying a style in darkroom view
so we dont just dont wipe the latest changes for a user...

Fix for bug #397
Fixes mouse coords translations for latest changes
of the filmstrip...
Initial folders module