Implement tabs in Thunar (bug #6102).

thunar/thunar-abstract-icon-view.c

thunar/thunar-application.c

thunar/thunar-device.c

thunar/thunar-launcher-ui.xml

thunar/thunar-launcher.c

thunar/thunar-navigator.c

thunar/thunar-navigator.h

thunar/thunar-preferences-dialog.c

thunar/thunar-preferences.c

thunar/thunar-shortcuts-pane.c

thunar/thunar-shortcuts-view.c

thunar/thunar-standard-view-ui.xml

thunar/thunar-standard-view.c

thunar/thunar-tree-pane.c

thunar/thunar-tree-view.c

thunar/thunar-window-ui.xml

thunar/thunar-window.c