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 implemnts 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 : introduce history type

This doesn't handle whole history, but history elements.
history elements are seen as tables with three fields, num, op and name.

all fields are read only

image object gain two fields
image:reset() will reset the history (i.e remove all processing)
image:get_history will return the history object for the image
lua tmp : introduce styles
everything else