Bug 75988 [Preferences] IPreferencePage#performOk() returning false should prevent the dialog from closing
I think that if this method returns false, it should prevent the dialog from closing. Today, in order to perform validation I have to register key listeners to all fields an use #setValid(false) to disable the "OK" button.
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferenceDialog.java
* @return the preference mananger
	 */
	public PreferenceManager getPreferenceManager() {
		return preferenceManager;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.jface.preference.IPreferencePageContainer#getPreferenceStore()
	 */
	public IPreferenceStore getPreferenceStore() {
		return preferenceStore;
	}
	/**
	 * Get the name of the selected item preference
	 * 
	 * @return String
	 */
	protected String getSelectedNodePreference() {
		return lastPreferenceId;
	}
	/**
	 * @param selection
	 *            the <code>ISelection</code> to examine.
	 * @return the first element, or null if empty.
	 */
	protected IPreferenceNode getSingleSelection(ISelection selection) {
		if (!selection.isEmpty()) {
			IStructuredSelection structured = (IStructuredSelection) selection;
			if (structured.getFirstElement() instanceof IPreferenceNode)