#176 Arrow key bindings to move selection

[Toby Cubitt]

It would be very useful to be able to use the arrow keys to move the current selection, in addition to click-hold-drag with the mouse.

This would make it much easier to make small, precise adjustments to the position of elements. It would also make it simple to move the selection only horizontally or only vertically. (Both are difficult with the mouse.)

Move-selection with arrow keys is the one key binding I miss badly from e.g. Powerpoint.