Add "disabled" state to Tree-Checkbox CSS element

Grid has a special API (GridItem#setCheckable), used by Nebula Grid, for
disabling the user iteraction with the checkbox. We need a coresponding
CSS state in order to have a different appearance for not checkable
checkboxs (hide checkbox image for example).

Change-Id: I95eea1b00258293b908f652be827d1544d48adeb
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>