2 steps required:

1. Refactor > Move > \Piwik\Core\Config (uncheck "Create new File")
2. Refactor > Rename > Piwik_Config to Config

Piwik\Core
Piwik\Plugin

Refactor Piwik_PluginsManager and Piwik_Plugin class, moved plugin metadata loading to a new MetadataLoader class, allowed loading of new colors.piwik.json file.

Fixes #4046 Not checking for config/config.ini.php in the system check since it's not required (and we display warning on other screens that need config writable)
Also fixing install process regression and removing config.ini.sample.php

Refs #546 Adding core "Uninstall" feature for Plugins and Themes + Cleanups + fix build by moving constant to PluginsManager
Todo
 * ask for confirmation: "Do you want to uninstall X?" // warn that plugin-specific data may be deleted as part of calling the plugin's uninstall method, and may not be recoverable.
 * call uninstall() method on the plugin class
 * verify core plugins cant be uninstalled

Modify unit tests expected values for browser detection

Fixes #4047 Just removing the test as it is actually not useful anymore (the manifest.inc.php test that follows is doing the job). thanks for the report!

 * Enabling one theme a time: automatically deactivates another theme if it was enabled (except Zeitgeist which is always on)

 * Zeitgeist is the foundation of our vision for an Open, Extendable Analytics Platform: Piwik 2.
 * This is the default theme for Piwik 2 from which other themes automatically inherit.
 * Plugins do not need a PluginName.php class anymore: plugin can be just a theme! (a json file + less/css/templates overloads)
 * Displaying themes Zeitgeist and PleineLune in the Themes listing

 * files I forgot to check in earlier

 * Removing PluginDescription from core en.php files, for all Example* plugins (to showcase the JSON file in the Example plugin)

 * Removing getInformation() call from core plugins
 * Renamed translation key PDFReports_PluginDescriptionReports => PDFReports_PluginDescription

Refs #3942 Adding Themes menu in the admin screen. Next: make Zeitgeist the default theme (and allow only one enabled theme at a time)

Fixed regressions in main menu selection: first menu item in submenus were never highlighted when first item is shown and all submenu items were highlighted when any other item is shown.

CSS, templates improvements. Enable Assetmanager on login page, separate css for installer/upgrader, progress in theming, Less installation. Super boulot Thomas !

Refactored ViewDataTable::getRequestString to return an array so building a URL is not necessary. Also changed request_string_suffix view property to request_parameters_to_modify.

Refs #4041, fix bug in displaying DBStats reports by redefining how columns_to_display view property defaults and allow unique visitors to be displayed on reports that have it, outside of period=day.