[Plugin Manager] New GUI
[Plugin Manager] Make QgsPluginManager instance persistent in QgisApp
[Plugin Manager] Remove obsolete class QgsPluginItem and use QgsPluginMetadata instead
[Plugin Manager] A new class QgsPluginMetadataRegistry. QgsPluginMetadata renamed to QgsPluginRegistryItem for unambiguous nomenclature.
Now QgsPluginRegistryItem is used in QgsPluginRegistry and QgsPluginMetadata in QgsPluginMetadataRegistry.
PluginManager moved to a subdirectory.