window-list: Restrict right-click actions to windows in list

While the list of windows in the left-click menu is filtered by
workspace, the minimize/maximize/close actions in the right-click
menu apply to all application windows on all workspaces.
This is fairly confusing, so restrict the actions to only apply
to windows that do appear in the left-click list.

https://bugzilla.gnome.org/show_bug.cgi?id=724134
window-list: Filter skip-taskbar windows

Applications now track all their windows, not just the ones that are
expected to show up in the window list. So to restore the previous
behavior, we now have to filter out windows with the skip-taskbar
hint ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=724134