Move several IPv4 and IPv6 specific procedures into separated newly

Move several IPv4 and IPv6 specific procedures into separated newly
created files ipv4.tcl and ipv6.tcl.

Move the MTU rolobox in interface config window from IPv4 address to toplevel.

Change the default IPv6 subnet from fefe:0:: to fec0:0:0:0:: which should
be more correct according to certain authors (site-local prefix space).

Add proc $MODULE.layer for each node type.  Return value can be either
NETWORK or LINK and is to be used primarily by editor.tcl for choosing
which configurable parameter to show / offer on a per-node basis.

Add proc removeNode and removeLink which do not manipulate on any Tk
objects, thus can be invoked in plain tcl scripts.  For removing Tk objects
(primarily in editor.tcl) removeGUINode and removeGUILink should be used.

Node names can now be arbitrary strings, including punctuation and
whitespaces.

Bug found by:
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