Plugin execution

[pmeijer]

The breaking change is that plugins are now required to have an associated metadata.json and the execution of browser plugins (once configured) are now handled by the client.

This PR deals with the following:

• Enables plugins to be executed under a namespace, i.e. the this.META within the plugin can be configured to contain only nodes under a given namespace (within a library). This allows the meta helpers function on PluginBase to adapt to the namespace.
• Plugin metadata introduces new key requiresWriteAccess - which prohibits execution of a plugin if the invoker does not have write access to the project. (The UI disables these plugins and the plugin manager throws an error if requirement not met.)
• Plugin configuration parameters are also extend with requiresWriteAccess. They are handled in the same way as above. (readOnly parameters are ensured to not be overwritten by the users too.)
• Execution of plugins in the browser share the same code base as for the server execution. Additionally there is no need to use the InterpreterManager anymore (it is still there but only deals with configuring plugins) - instead the client has a method runBrowserPlugin with the same parameters as runServerPlugin.

[kecso]

writes -> rights