Add mate-gsettings-toggle.sh

In gsettings there is no toggle option which mateconf-tool did have. We need this to fix https://github.com/mate-desktop/mate-settings-daemon/issues/32 by replacing mateconftool-2 --toggle with this script.

This little shell script can toggle the value of 3 keys in monitor org.mate.applications-at

screen-keyboard-enabled
screen-magnifier-enabled
screen-reader-enabled

I intentionally limited it to these keys to make it easier to check the values provided to the script.

Next is to update the schema in mate-settings-daemon, org.mate.keybindings.gschema.xml.in.

___This commit should be released at the same time as https://github.com/mate-desktop/mate-settings-daemon/pull/49___