obj_* function naming conventions


High level (new/destroy):

	This is the recommended way of creating new objects.

	pcb_*_new():
		Calls pcb_*_alloc(), fills in fields from parameters and calls the post
		processing (pcb_add_*_on_layer()) function as needed. Makes sanity checks
		on the parameters and may fail upon that. 

	pcb_*_new_*():
		Same as pcb_*_new(), but may do clever things so the object created may
		not be 1:1 what was requested or may not even be a new object (e.g.
		overlapping line merging)

	pcb_*_destroy()
		free all fields of an object and properly remove it from the parent
		(including rtree removal)

Low level (alloc/free):

	Use this in some special cases (like loading a file) when the extra checks
	of a _new may interfere. 

	pcb_*_alloc():
		allocate a new objects within a parent (as of now: layer). Allocates
		the struct of a new objects with all fields clean and links the object
		in the parent list. Returns a pointer to the object. NOTE: some post
		processing may be needed after filling in the fields
		(see also: pcb_add_*_on_layer()). Calls pcb_*_reg() automatically.

	pcb_*_alloc_id():
		same as pcb_*_alloc(), but instead of automatically pick the next id,
		uses the user supplied ID

	pcb_*_free():
		free the struct memory (but not the fields!) of an object and remove
		it from the parent list. Calls pcb_*_unreg() automatically.

	pcb_*_reg():
		register an object in data; this includes adding the object in the right
		object list and in the id hash, but does not include any rtree operation
		(so coords/sizes may change later). Also sets the object's parent.

	pcb_*_unreg():
		unregister an object from its data; this includes removing the object from
		the right object list and from the id hash, but does not include any
		rtree operation.  Also clears the object's parent.

	pcb_add_*_on_layer():
		Postprocessing: call this after a new object is allocated and fields are
		filled in. It mainly updates the rtree.

	pcb_*_copy():
		copy fields of a complex obj to a pre-allocated target obj

	pcb_*_pre() and pcb_*_post():
		when an object needs to be moved/changed within its own context (e.g. layer
		object moved within layer, global object moved within data), it first needs
		to be removed from the rtree and uncleared from any sorrunding polygon.
		After the changes are done, the object should be put back in the rtree and
		the affected polygons should be cleared. These two steps are done by
		pcb_*_pre() and pcb_*_post(). Call pre() before the operation, call post()
		after the operation.

Accessors:
	pcb_*_get_*():
		Return data that are not directly accessible as object struct fields but
		require some sort of calculation

	pcb_*_set_*():
		Set object fields that require postprocessing because of side effects.
		Use these instead of direct writes to the fields to get side effects
		properly executed.

	pcb_*_change_*():
		Similar to pcb_*_set_*(), but instead of setting a field to a value
		specified as a parameter, change the field to comply with some global
		state, e.g. "change side" would move the object to the currently viewed
		side.

Transformations:
	pcb_*_move():
		move the object within its parent (x;y); it's often implemented as a
		macro

	pcb_*_mirror():
		mirror the object within its parent (x;y)


Misc:
	pcb_*_bbox():
		calculate and update the bounding box of an object

	pcb_*_rotate90()
		Rotate the object by n*90 degrees