Added events for Piwik update, enable/disable plugin and Plugin Settings update

This is needed for #6372

New events:

- `CoreUpdater.update.end`

    Triggered after Piwik has been updated.

- `PluginManager.pluginActivated` - Parameters: `$pluginName`

    Event triggered after a plugin has been activated.

- `PluginManager.pluginDeactivated` - Parameters: `$pluginName`

    Event triggered after a plugin has been deactivated.

- `Settings.$settingsName.settingsUpdated`

    Triggered after a plugin settings have been updated. - Parameters: `Settings $settings`

    Example:

    ```php
    Piwik::addAction('Settings.MyPlugin.settingsUpdated', function (Settings $settings) {
        $value = $settings->someSetting->getValue();
        // Do something with the new setting value
    });
    ```