Bug 73083 [Encoding] Need to verify encodings and indicate unavailable ones
When the encodings are set up we need to ensure that they are in the list of valid encodings available from the VM. Those that are not should be indicated as such and not be selectable in the UI.
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/ide/dialogs/AbstractEncodingFieldEditor.java
* @return String
	 */
	protected abstract String getStoredValue();
	/* (non-Javadoc)
	 * @see org.eclipse.jface.preference.FieldEditor#doLoadDefault()
	 */
	protected void doLoadDefault() {
		updateEncodingState(true);
	}
	/* (non-Javadoc)
	 * @see org.eclipse.jface.preference.FieldEditor#getNumberOfControls()
	 */
	public int getNumberOfControls() {
		return 1;
	}
	/* (non-Javadoc)
	 * @see org.eclipse.jface.preference.FieldEditor#isValid()
	 */
	public boolean isValid() {
		return isValid;
	}
	/* (non-Javadoc)
	 * @see org.eclipse.jface.preference.FieldEditor#refreshValidState()
	 */
	protected void refreshValidState() {
		updateValidState();
	}
	/* (non-Javadoc)
	 * @see org.eclipse.jface.preference.FieldEditor#setPreferenceStore(org.eclipse.jface.preference.IPreferenceStore)
	 */