The 'mode_sigh' function allows the client to receive notifications
about server-side display-mode changes. To respond to such a signal, the
client can use the new 'release' function, which acknowledges the mode
change at the server and frees the original framebuffer dataspace. Via a
subsequent call of 'dataspace', a framebuffer dataspace corresponding to
the new mode can be obtained. Related to issue #11.
As a preliminary step for working on issue #11, this patch revisits the
'Framebuffer::info' RPC call. Instead of using C-style out paramters,
the new 'mode()' RPC call returns the mode information as an object of
type 'Mode'. Consequently, mode-specific functions such as
'bytes_per_pixel' have been moved to the new 'Framebuffer::Mode' class.
