gtk: allow moving tabs to windows and back.

gtk: allow moving tabs to windows and back.

"View->Detach tab" will move to tab to a new window.
Simply closing the window will move it back into a notebook tab.
The label will be permamently stored in VirtualConsole->label,
so it can easily be reused to (re-)label tabs and windows.

Works for vte tabs only for now. pointer/kbd grab code needs
adaptions before we can enable it for gfx tabs too.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>