Add a GUI element allowing to manage plugins

Current situation:
Plugins are only loaded at startup by scanning all the ones installed in the userDataDir/modules/ and looking if the load_at_startup option = true in the module.ini
To activate or deactivate a plugin, the user needs to edit this file by hand and toggle this boolean. Then to restart the program.

Requirements:
Have a new window listing the available plugins with associated informations and allowing to activate or deactivate them. Some plugins may require a restart of Stellarium to be activated or deactivated.
It is not yet clear where to put the window in the GUI.

At the current time, we don't plan to allow download or automatic update from internet. All the plugins can be assumed to be distributed along the main package.

Skills needed: C++, Qt, GUI Design
Difficulty: Not very difficult