Work on #256: implement virtual arrow key handling

Remaps can now send <UP>, <LEFT>, <RIGHT>, <DOWN> in insert mode and
expect to see them handled. Previously they were outright ignored.
Fix #256: update sticky column only for left/right

Moving up would always lose the "sticky column", making the feature
useless.

Note that pressing the arrow keys might use a different sticky column
than when using a map for <LEFT> / <RIGHT>. This is because the arrows
are sent to Eclipse whereas L/R is handled by Vrapper.