Add origin-shifting (Shift+cursors) and source-shifting (Ctrl+cursors) to Net.

Add origin-shifting (Shift+cursors) and source-shifting (Ctrl+cursors) to Net.
(Adding modifier+cursors handling has had minor knock-on effects on the other
puzzles, so that they can continue to ignore modifiers.)

(An unfortunate side effect of this is some artifacts in exterior barrier
drawing; notably, a disconnected corner can now appear at the corner of the
grid under some circumstances. I haven't found a satisfactory way round
this yet.)

[originally from svn r5844]