Fix dragging objects on the canvas and remove redundant canvas groups

Delivery of fake motion events to the editor needed the event coordinates to be
in canvas space, as they are with "real" events. Editor and other objects had
many redundant groups from timbyr's work on gnomecanvas to scroll by moving
groups. We don't need this anymore with cairo-canvas (though possibly a
stationay background group for the canvas might be useful again one day as in
the SAE logo. Its implementation would be fairly different though, since we
would have to explicitly move the group on every scroll, since nothing else
ever moves on scroll).

Also tweaks to text item placement, and switch TimeAxisViewItem from
name_pixbuf to name_text, since ArdourCanvas::Text is already "pixbuf optimized".