Bug 181308 [Viewers] Add checked state provider to CheckBox*Viewer
It would be nice to have a provider (similar to the label or font or color providers) to determine the checked/grayed state of elements in a CheckBox*Viewer (either Table or Tree). This would generally simplify the programming model and give the developer better control over how the model objects are displayed in the viewer.
bundles/org.eclipse.jface/src/org/eclipse/jface/viewers/CheckboxTreeViewer.java
* @param parent the parent control
     */
    public CheckboxTreeViewer(Composite parent) {
        this(parent, SWT.BORDER);
    }
    /**
     * Creates a tree viewer on a newly-created tree control under the given parent.
     * The tree control is created using the given SWT style bits, plus the <code>CHECK</code> style bit.
     * The viewer has no input, no content provider, a default label provider, 
     * no sorter, and no filters.
     *
     * @param parent the parent control
     * @param style the SWT style bits
     */
    public CheckboxTreeViewer(Composite parent, int style) {
        this(new Tree(parent, SWT.CHECK | style));
    }
    /**
     * Creates a tree viewer on the given tree control.
     * The <code>SWT.CHECK</code> style bit must be set on the given tree control.
     * The viewer has no input, no content provider, a default label provider, 
     * no sorter, and no filters.
     *
     * @param tree the tree control
     */
    public CheckboxTreeViewer(Tree tree) {
        super(tree);
    }
    /* (non-Javadoc)
     * Method declared on ICheckable.