lua : add events

darktable.register_event(event_type,callback,...)

this function register a callback to be called when a given event happents

* event_type : the event to register to
* callback : the function to call on event
* ... : extra parameters, depend on the type of event

this implements two events

* post-import-image : no extra parameters
	called when an image is imported into the database, before any processing happens. Can be registered multiple times
* shortcut : one extra parameter : string : name of the shortcut in the shortcut panel
	called when the corresponding shortcut is pressed. can only be registered once per shortcut
lua : add a way to get the hovered image and the action_images

dt.gui.hovered : the id of the hovered image or nil if no image is hovered
dt.gui.action_images : a table of images on which the user expects us to act.
	It is based on both the hovered image and the selection and is
	consistent with the way darktable works
gettext: fix but that was resetting purple color label collections in non-english language.
Fix Lightroom flip orientation.

Do not try to use the flip from the crop iop as it seems to be not
working in some cases. Instead of beeing smart we now simply use the
orientation module to rotate the image if needed. This anyway simplify
the code. Also make sure that the atof() is locale independant as
lightroom always store the float values with a dot as decimal
separator.
Don't mess with the locale when parsing strings

Changing the locale to make atof treat '.' as the decimal separator is
not thread safe and therefore to be avoided!
Merge pull request #241 from darktable-org/lua_review

lua : add events
(cherry picked from commit 85a44115d007e4081eb8ef4f5502a2f0277ad1dd)

Conflicts:
	src/common/exif.cc
Merge branch 'master' of https://github.com/darktable-org/darktable
lua : add a way to export images directly with format objects

<format>.write_image(format,image,filename)

will export the image to filename using format.

this call is blocking. Another function will be provided to queue images at some point
mask manager : add a way to cleanup unused shapes
image information: double click on film roll jumps to that as collection.
jump-to now with key accel (ctrl-j)
recentcollect: now also remembers position in collection.
recentcollect: accel ctrl-k to jump back to previous collection.
masks path : enhance size decreasing
Honor the selection or the image hovered when exporting with ctrl-e

When exporting with ctrl-e the selection or the image hovered
are the one exported following the same rules than for rating
or assigning stars
noise profile: canon eos-m by christoph
[l10n] Fix some translation issues in our code

Sorry, this might break some old key accels. We should have done that
correctly in the first place ...
new lines added, a few lines modified to match context (mask operations)
Merge remote-tracking branch 'upstream/master'