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