[Plugin Manager] Store metadata in QgsPluginMetadataRegistry instead of in item model
[Plugin Installer] Move the plugin installer code from plugin to core
[Plugin Manager] QgsPluginManagerInterface class and basic communication Plugin Manager <-> Plugin Installer
[Plugin Manager] Revert QgsPuginMetadata and QgsPluginMetadataRegistry
[Plugin Manager] New metadata in Python part
[Plugin Manager] Rename the plugin_installer python module to avoid conflicts with the old plugin_installer plugin
[Plugin Manager] Big code cleanup
[Plugin Manager] add metadata description file