core: Only trigger MetaWorkspace::window-* on toplevel window types
Popups and other override-redirect windows are irrelevant to all listeners of MetaWorkspace::window-*. Ignoring those windows will namely result in less activity on ShellWindowTracker, less ShellApp::windows-changed signal emissions and less AppMenuButton updates.
Reduces gnome-shell CPU activity while typing on the Epiphany addressbar, as the pop up animation there results in a number of xdg_popup being created and destroyed.