Pcbnew: fix Bug #1304418 . Fix also a related issue which crashes Pcbnew in some corner cases.
(for instance, when a footprint from the currently edited board is loaded in the footprint editor, and when the board is cleared or reloaded)
In footprint editor, the net names are no more shown od modifiable (becuase the footprint editor does not know anything about net names.
This change should allow the changes planned in pcbnew.