Feature plugin manager

This includes a bit of refactoring of the `Minimap` class (see the first commit).

Now the views management is handled in a mixin. As for the main purpose of this PR, the minima plugins management.

The basic feature is that the minimap module now know about plugins, and provides settings to enable/disable them. 
The second feature is that the minimap can activate and deactivate plugins independently of the plugin's package activation. 
This feature are interesting because they allow us to provides GUI about the minimap and its plugins (as a contextual menu to activate/deactivate plugins).

And these features are tested (there's still more cases to cover though).

As a side note, I already have the find-and-replace plugin ready to use the plugins API, so tell me when you publish the version that will include this feat so that I can publish the plugins sing the new API.