Remove some unnecessary returns.

Found by Coverity Scan.
BusyCursor: Use the non-deprecated Gdk::Cursor::create(), taking a Display.
DbAddDel: Do not use deprecated set_rules_hint().

Apparently this is now for the theme to decide.
CanvasItemMovable: Create the drag cursor for the specific display.

This avoids using the deprecated Gdk::Cursor constructor that
doesn't take a Gdk::Display.