[compiz] invalid binding strings

[moppsy at comcast.net]

gconf places no restrictions on setting string values.
compiz does not seem to validate binding strings and I have noticed that it can cause quirky behavior.

The exact situation that brought this to my attention was setting opacity_increase and opacity_decrease to empty string values.  After initiating switcher with Alt-Tab, while keeping Alt pressed, pressing shift changes the active windows opacity.   Others have verified this, and I can confirm that setting the same options to other invalid binding strings also causes this behavior.

Others have expressed that I must enter disabled into gconf because it is required.  It is my opinion that removing the binding in gconf should disable it without forcing the user to have to type disabled in its place.