Get rid of meta registry
Store window registries in objreg.
Fix objreg._get_window_registry when non-mainwindow window is focused.
Fix closeEvent of MainWindow.
Revert "Store window registries in objreg."

This reverts commit dfd3b3d9c446601fa30ba94488b683a72dd25673.

It turns out this makes it very hard to remove the window from the window
registry at the right time.
Register tab registry under window registry with ID.
Fix wrong 'is' check.
Print tab registries in :debug-all-objects.