Do not modify associated device for non-master devices on dispose

Do not modify associated device for non-master devices on dispose

Only master devices must modify the associated device to separate
a pointer/keyboard pair, slave devices must only call
_gdk_device_remove_slave().

Fixes bug 639767 - password not accepted in gnome-screensaver dialog,
reported by Frederic Crozat. On VT-switch, the X server removes its
grab on HW devices, the effect on clients is that slave devices
disappear, and these were mistakenly mangling the master device
hierarchy. so gdk_device_get_associated_device() on the client
pointer wouldn't return the paired keyboard anymore.

The final effect is that gtkplug-x11 wasn't setting a keyboard to
its generated events.