Silent plugin installation

The aim of this functionality is to make the process of installing plugin updates and new recommended plugins easier to the user, so that it doesn't get in her way.

Currently the user can be disturbed by several plugins-related events in the form of an informative popup:
 - All recommended plugins successfully installed (first run after installation on windows), need to restart Elisa;
 - Plugin updates available, download and install them?;
 - Plugin updates complete, need to restart Elisa;

We also want to add the detection of new recommended plugins at startup and periodically (with the check for plugin updates), that would potentially add a new popup asking the user whether she wants to install new recommended plugins.

The simplification is twofold:
1) Queue and group notifications as much as possible.
2) Add configuration parameters that allow silent installation of new recommended plugins/plugin updates without asking the user's confirmation.