Limit resolutions

In the config, the user specifying scales. The OpenStreetMap or Google
Maps layers have their own scales. The scales specifying by the user is
used to limit the number zoom levels of these layers.
User defined Client cache browser-side for layers

Set the expiration time in seconds if set by the plugin.
This improve perfs a lot, because no request is made by the client for the same image if the browser has it in Cache and the image has not yet expired