inherit parent visibility and pinned status

When in "only show pinned windows" modus and opening a context menu on a pinned window by right clicking, the child window was hidden, because by default new windows are not pinned.

This MR changes the behavior so that newly opened child windows inherit whether they are pinned from their parent windows, as well as the current visibility.

I noticed that xrd_window_is_selected, which I first wanted to inherit too, has no users and can be deleted.