New Group object

[Lars Clausen <lars raeder dk>]

One of the two goals I have for the next version of Dia is a grouping
system that doesn't suck.  The current one sucks in the following ways:

* You cannot 'open' a group to edit the elements inside.
* You cannot have connections into or out of a group -- in fact, if you
group a connection or a connected object, connections are *broken*
* Grouping destroys the height information of the objects, they all end
up at one level no matter what they were in front of or behind.

So I've added a new object under the Misc sheet for testing a new group.
The interface so far is nothing like the old group, in fact it's just
parenting, but that's because parenting and grouping now becomes even
more related.  So if you make some objects children of the NewGroup
object, they should behave as in a group, but with the ability to 'open'
and 'close' the group through group properties.  What I'm trying to
ensure right now is that the various ways to interact with grouped
objects makes sense -- you shouldn't be able to select an object in a
closed group, or type to one, or basically interact with it in any way.

Once the interactions are dealt with, I will make it look like the
regular group and have it take that place.

-Lars