[Merkaartor] Virtual nodes

[Toby Speight]

I've just synced up to SVN and I like the new 'virtual node' concept.
This seems to address my wish for faster refinement of existing ways.

Unfortunately, it seems to bring down Merkaartor with a segfault if I
accidentally hit one of these virtual nodes whilst adding a new way;
possibly also in other circumstances.  Turning off virtual nodes in the
View menu doesn't help, as they remain selectable and therefore at risk.

I'd suggest that virtual nodes not be selectable except in Move mode if
enabled, Select mode otherwise.

I'm hoping to hear that this is a work in progress and this issue is
known, but just in case...