Add possibility to read more than one local plugin descriptors for the dashboard

Current situation: org.onehippo.cms7.essentials.plugin.PluginStore reads local file "plugin_descriptor.json" and descriptors configured in the project-settings.xml in the node <pluginRepositories>.

The value of that is supposed to also read local descriptors on the classpath with a "classpath:" prefix, but I failed to get that to work. Only if the path configured is a complete classpath (so including path to the project) from the root it works. For example:

<pluginRepositories>file:/Users/myname/code/training/myhippoproject/essentials/extra_plugins_descriptor.json</pluginRepositories>

Ideal new situation: just drop the extra descriptor in a specific folder of the essentials project, where it will be read by the PluginStore class also, next to the default one.