Recommended / Stable / Unstable plugins

We want to be able to make a distinction between stable and unstable (that is, in development) plugins, so as to allow distribution of plugins before they are actually fully reviewed and tested. This will allow a more dynamic process of publication for the plugins and hopefully encourage contributors to write plugins.
We also want to be able to promote plugins as recommended when we judge they improve visibly the user experience.

The idea is to have an "advanced/power user" configuration option, disabled by default, that, when enabled, will give more control over Elisa, such as listing unstable plugins in the "Find New Plugins" section.

We also need to implement a way to mark plugins as recommended/stable/unstable server-side, in the plugin repository.