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