xdnd: Don't wait for the stage to be disabled when it is already visible

xdnd: Don't wait for the stage to be disabled when it is already visible

xdndHandler used to wait for the stage to show up before calling
global.init_xdnd() . Since commit 65303d027aa8 xdndHandler is initalized
later so the stage might be already visible at this point causing the
show signal never to be emitted and thus global.init_xdnd will never
be called.

Fix that by checking by calling global.init_xdnd without waiting for
the stage's show signal to be emitted, as the stage is guaranted to be
visible at this point.