Layer groups: more discoverable, less weirdness

Make layer groups more discoverable: additional New Layer Group commands in
both the context and main menus.

In the layers panel, relegate the drag-layer-into stuff to dragging with
shift held down. It's weird (though IMO cool) behaviour, so make it
non-default. Add statusbar hints about it, because power-user stuff
shouldn't be totally hidden.

Fixes #68.