When a child window of a fullscreen client is mapped, the fullscreen is disabled, and if the previously fullscreen client is floating the child window is rendered below it and cannot be seen, causing confusion, though it is still focused and interactable.

Fix this by putting children of fullscreen clients in LyrFS instead of LyrFloat, and by returning before the unset_fullscreen code is called when they are mapped.

focusstack() now lets you switch focus from a fullscreen client to its child windows, otherwise if you switch focus from the child window to the fullscreen client you could not focus the child window again and the fullscreen client would stay unresponsive.

Child clients are not reparented to LyrFloat after leaving fullscreen, so you could spawn a child window, focus back the fullscreen client, unfullscreen it, and the child window would still be drawn above other floating clients. Avoid dealing with this edge case to keep the line count low.

These cases can be tested by pressing Ctrl+o in applications with an open file dialog.