Option to set/change plugin configuration

[chapulina]

Currently, support for plugin configurations is limited to passing them at the moment the plugin is loaded. This configuration can only be passed programmatically or from the command line. This makes it impossible to configure a plugin that's inserted from the top-right menu, for example.

It would be nice to add the ability of changing a plugin's configuration during runtime.

• On the GUI, this could be an extra field on the plugin's Settings menu.
• To make things simple, we could just let users input the config string in an XML format through the GUI. In the future, we could consider a way for plugins to declare their schema so we can generate an interface with fields to be filled.
• In the code, we could add a new optional callback for plugins to implement when they receive a new configuration, similar to LoadConfig. Maybe UpdateConfig, which receives an XML element with the new config.