Multiple matchbox-keyboard instances would occupy whole screen and cause X
window segfault. Making matchbox-keyboard singleton is one work around.
This patch allow only one instance of matchbox-keyboard based on some mechanism
in matchbox-desktop. In future, an applet and GTK-IM modules in
matchbox-keyboard can be used to automatically map/unmap the virtual keyboard
on demand.
[BUGID #509] got fixed.
Signed-off-by: Zhai Edwin <edwin.zhai@intel.com>