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
Merge pull request #241 from darktable-org/lua_review

lua : add events