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
if (isValidNow != isValid) {
			isValid = isValidNow;
			if (isValid) {
				clearErrorMessage();
			} else {
				showErrorMessage(IDEWorkbenchMessages
						.getString("WorkbenchPreference.unsupportedEncoding")); 
			}
			fireStateChanged(IS_VALID, !isValid, isValid);
			String newValue = getSelectedEncoding();
			if (isValid && !newValue.equals(oldSelectedEncoding)) {
				fireValueChanged(VALUE, oldSelectedEncoding, newValue);
				oldSelectedEncoding = newValue;
			}
		}
	}
	/**
	 * Get the encoding current selected.
	 * @return String
	 */
	protected String getSelectedEncoding() {
		if (defaultEncodingButton.getSelection()) {
			return defaultEnc;
		}
		return encodingCombo.getText();
	}
	private boolean isEncodingValid() {
		return defaultEncodingButton.getSelection() || isValidEncoding(encodingCombo.getText());
	}
	private boolean isValidEncoding(String enc) {