LayerControl 1.3.1 patch

Updates and fixes:

1. Add a base class for all controls.
2. Fixed bug in dynamic layer where toggling all sublayers on/off also affected other dynamic layer sublayer visibility.
3. All controls are aware of external changes to layer visibility and update accordingly.
4. Add `showAllLayers` and `hideAllLayers` methods to toggle all layers on/off.
5. Add control param `metadataUrl` to add Metadata link menu item to the layer menu.