cursor-shape: Add the 'ask' cursor shape

This is the cursor shape that corresponds to the ASK drag action
in the core protocol. The expected semantics of ASK are that the
drop target presents the user with a choice of actions when the
drop happens. A typical image for this cursor is a default cursor
with a '?' emblem.

Signed-off-by: Matthias Clasen <mclasen@redhat.com>