Make a typedef for shell creation return types

[Timidger]

Currently the shell return types are very long and awkward to type. It's clear what needs to be return, but it's very hard to read. E.g.:

pub fn new_surface(compositor_handle: compositor::Handle,
                   shell_handle: xdg_shell::Handle)
                   -> (Option<Box<xdg_shell::Handler>>, Option<Box<surface::Handler>>) {
    (None, None)
}

If we could condense that into something like this it would be much better (name undecided):

pub fn new_surface(compositor_handle: compositor::Handle,
                   shell_handle: xdg_shell::Handle)
                   -> XdgShellHandlers {
    (None, None)
}