These are cosmetic improvements spotted while working on the issue.
1. Do not waste time if there are no changes to notify about.
2. Fix the legacy plugin manager's get_plugins() call.
Credit goes to PJ King. I was trying to add unit tests for this new
behaviour but it turned out there are many hard-coded dependencies and
it's not easy to make the whole machinery support alternative location
of testable (fake) plugins.
Includes:
* update checker refactored to \core\update\ namespace
* plugininfo classes refactored to \core\plugininfo\ namespace
* plugin_manager renamed to core_plugin_manager
* redirect back to original page after plugin uninstall
* fixed assign subplugin uninstall
* move assign subplugins under the assignment in admin tree
* fixed plugininfo for all question related plugin types
* auth uninstall support
* added missing block dependencies
* added theme uninstall
* subplugin types are following the plugin on plugin overview page
* several performance improvements in plugin manager
* new warnigns when plugininfo are outdated or missing
* multiple fixes and other improvements
